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Peter M. Pryciak, Ph.D.

Assistant Professor of Molecular Genetics and Microbiology

Other UMMS Program Affiliations:
Cell Dynamics Research Group
Interdisciplinary Graduate Program

Signal Transduction and Cell Polarity

Photo: Peter Pryciak Research in my laboratory is aimed at understanding how cellular behavior is dictated by external signals. Currently, our studies stress three general topics: What is the role of subcellular compartmentalization in signal transduction?  How do cells judge the location of extracellular cues and mount a directional response?  How are different signaling pathways with shared components kept insulated from each other?

To investigate these issues, we use the simple eukaryotic cells of bakers yeast, Saccharomyces cerevisiae, as a model for more complex cells.  (see Figure 1 )

The role of subcellular localization in signal transduction has been highlighted by our efforts to understand how the heterotrimeric G protein Gbg complex activates its downstream MAP kinase cascade.  Here, our observations suggested that this activation involves the recruitment of a kinase cascade “scaffold” protein, Ste5, from the cell interior to the periphery.  Furthermore, our work suggests that the factor waiting for the MAP kinase cascade to arrive at the membrane is the PAK-family kinase Ste20, and that the function of Ste20 depends on another membrane-resident protein, the GTPase Cdc42.

A second project is focused on how cells polarize toward localized signals.  This behavior, exhibited by many types of cells, implies communication between the molecules that sense the signal and those that govern cell shape and polarity.  Our work suggests that, in addition to its role in activating the MAP kinase cascade, the yeast heterotrimeric G protein has a separate function in orienting cell polarity along gradients of pheromone chemoattractants.  This appears to involve the formation of multiprotein complexes between G bg and polarity-control proteins (see Butty et al, 1998).  In addition, our work suggests that Gbg must be regulated in a spatially-asymmetric manner by the receptor and G a subunit in order for to properly guide cell polarization along external gradients. (see Figure 2 )

We are also interested in the fidelity of signaling molecules, and have recently begun a project investigating the “insulation” of different signal transduction pathways.  In yeast, as in human cells, multiple independent signaling pathways operate to generate different responses to different stimuli.  Remarkably, individual protein molecules sometimes function in multiple pathways, raising the question of how these pathways are kept insulated from each other to avoid “crosstalk” between them.  We have been able to show that association with pathway-specific binding partners causes signaling molecules to become sequestered into discrete complexes that are dedicated to single pathways.  To do this, we developed a method for forcing signaling molecules to associate with a subset of their possible partners.  We anticipate that this approach will be of wide interest, as it allows one to create custom signaling molecules that are targeted toward chosen pathways. (see Figure 3 )


Figure 1

Figure 1.  Mating reaction (left) and pheromone response pathway (right) of Saccharomyces cerevisiae. Mating pheromones (a factor and a factor) secreted by each cell stimulate the partner cell to arrest cell division and alter cell polarity. Many proteins have been identified as signaling intermediates in pheromone response (right).  Our efforts have been directed at understanding the function of the G bg dimer, the “scaffold” protein Ste5, the PAK family kinase Ste20, and the control of polarity by the receptor/G-protein system.

Figure 2

Figure 2.  (left) Pheromone-stimulated membrane recruitment of the kinase cascade scaffold protein, Ste5.  This recruitment is dependent on the G bg dimer, but not on downstream signaling events, and is thought to allow the Ste5-associated kinase cascade to become activated by the membrane-bound PAK kinase, Ste20 (see Pryciak and Huntress, 1998). (right) Cartoon depicting yeast cells being exposed to a gradient of pheromone released from a micropipette.  Wild-type cells produce projections that grow in a directional fashion toward the source of pheromone (Segall, 1993).  Chemotropism mutants are unable to orient their projections along the pheromone gradient.  Defects in chemotropism result when the G bg-Far1-Cdc24 complex is disrupted by mutation in Far1 (Valtz et al, 1995; Dorer et al, 1995; Butty et al, 1998), Cdc24 (Nern and Arkowitz, 1998), or Gbg (Pryciak et al, in preparation).

Figure 3

Figure 3.  Distinct pathways share a common component. The kinase Ste11 functions in at least three separate signaling pathways: mating, filamentous growth, and the high osmolarity glycerol (HOG) response.  Though sharing a common kinase, these pathways are insulated from each other, as each stimulus activates only a single pathway.  Pathway-specific scaffold proteins such as Ste5 and Pbs2 are thought to provide this insulation.  In addition to binding kinases, these scaffolds bind pathway-specific activators at the membrane, possibly coupling each stimulus to a discrete set of kinases.  Membrane recruitment of Ste5 by Gbg is implicated in the activation mechanism of the mating pathway (Pryciak and Huntress, 1998); a related role for interaction of Pbs2 with the transmembrane protein Sho1 has been recently proposed (Raitt et al, 2000).  Note that Ste20 can function in all three pathways as the activator of Ste11; the suggestion that stimuli bring select substrates to Ste20 (Pryciak and Huntress, 1998) may explain how Ste20 avoids cross-talk.  Whether a scaffold exists for the filamentation pathway is unknown.

Recent Publications

Lamson, R.E., Winters, M.J., and Pryciak, P.M. (2002).  Cdc42 regulation of kinase activity and signaling by the yeast PAK Ste20.  Mol. Cell. Biol.  22: 2939-2951.

Harris, K., Lamson, R.E., Nelson, B., Hughes, T.R., Marton, M.J., Roberts, C.J., Boone, C., and Pryciak, P.M.  (2001).  Role of scaffolds in MAP kinase pathway specificity revealed by custom design of pathway-dedicated signaling proteins.  Curr. Biol.  11: 1815-1824.

Moskow, J.J., Gladfelter, A.S., Lamson, R.E., Pryciak, P.M., and Lew, D.J. (2000). Role of Cdc42p in pheromone-stimulated signal transduction in Saccharomyces cerevisiae.  Mol. Cell. Biol. 20: 7559-7571.

Pryciak, P.M. and Huntress, F.A. (1998). Membrane recruitment of the kinase cascade scaffold protein Ste5 by the G bg complex underlies activation of the yeast pheromone response pathway. Genes Dev. 12: 2684-2697.

Butty, A.-C., Pryciak, P.M., Huang, L.S., Herskowitz, I. and Peter, M. (1998). The role of Far1p in linking the heterotrimeric G protein to polarity establishment proteins during yeast mating. Science 282: 1511-1516.

Pryciak, P.M. and Hartwell, L.H. (1996). AKR1 encodes a candidate effector of the Gbg complex in the Saccharomyces cerevisiae pheromone response pathyway and contributes to control of both cell shape and signal transduction. Mol. Cell. Biol. 16: 2614-2626.

Dorer, R., Pryciak, P.M. and Hartwell, L.H. (1995). Saccharomyces cerevisiae cells execute a default pathway to select a mate in the absence of pheromone gradients. J. Cell Biol. 131: 845-861.

Rotation Projects

Project #1: How the heterotrimeric G protein bg complex (Gbg) activates a downstream MAP kinase cascade. We know that G bg stimulates the translocation of a kinase cascade "scaffold" protein, Ste5, to the plasma membrane. This project would address whether factors in addition to Gbg can contribute to recruitment or retention of Ste5 at the plasma membrane; and if so, identifying these factors by a genetic approach.

Project #2: How cells polarize toward localized signals. In addition to activating the MAP kinase cascade, the yeast G bg complex has a separate function in orienting cell polarity along gradients of chemoattractant pheromones, by complexing with cytoskeletal proteins. This project would address how the receptor/G-protein system organizes a focus of morphogenesis to a discrete region of the cell periphery.

Project #3: The function of a novel transmembrane G bg-binding protein, called Akr1. We suspect Akr1 helps traffic the Gbg complex to and away from the plasma membrane. This project would use fluorescence microscopy to determine the subcellular location of Akr1, which is likely to be Golgi or endosomal membranes. A related project would test the role of endocytosis in regulating Gbg signaling or desensitization.

Academic Background

B.S., University of California, Los Angeles, 1983.
Ph.D., University of California, San Francisco 1992.

Postdoctoral Fellow, University of Washington, Seattle, 1992-1996.

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Office: B4-334
Phone: (508) 856-8756
Keywords: Signal Transduction, Cell Biology, Genetic Systems

Postdoctoral Position Available

A postdoctoral position is available to study in this laboratory. Contact Dr. Pryciak for additional details.

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