Abstract #92B

Phosducin-like Proteins as Modulators of the Chaperonin CCT. Peter Stirling¹, Karam Takhar¹, Srayko Martin², José Valpuesta³, Ronald Melki⁴, Michel Leroux¹. 1) Dept Molec Biol & Biochemistry, Simon Fraser Univ, Burnaby, BC, Canada; 2) Max-Planck-Institute of Molecular Cell Biology and Genetics, Dresden, Germany; 3) Centro nacional de biotecnologia, Campus autonoma de universidad de Madrid, Spain; 4) Laboratoire d’enzymologie et biochimie structurales, CNRS, Paris, France.
   The chaperonin CCT is an essential protein folding machine that may be responsible for folding ~10% of all eukaryotic cytosolic proteins, including its best-characterized and highly abundant substrates, actin and tubulin. CCT function is known to be modulated by its co-chaperone prefoldin and was recently found to be modulated by a family of evolutionarily conserved Phosducin-like proteins (PhLPs). PhLP1 was shown to inhibit substrate folding by CCT in 2002 and we have since discovered another PhLP homolog, PhLP3, that interacts with CCT and modulates its function. PhLP3 forms a ternary complex with CCT and substrate protein, slowing ATP hydrolysis by the chaperonin. Consistent with this observation, we observed that excess PhLP3 inhibits protein folding in vitro but does not prevent substrate binding to CCT. Our studies show that in vivo, S. cerevisiae PhLP3 (termed PLP1) is important for the formation of both a functional microtubule and actin filament network, consistent with a modulatory rather than strictly inhibitory effect on CCT. Interestingly, PLP1 is non-essential and the phenotypes observed are only clearly apparent in strains also lacking a functional prefoldin complex. Indeed, prefoldin and PhLP3 appear to have antagonistic effects on the CCT functional cycle which lead to the optimal production of cytoskeletal proteins. Given the above findings, it is intriguing that the other yeast Phosducin-like protein, Plp2, is essential for viability. We have found that Plp2 also binds CCT and affects cytoskeletal processes. Our characterization of temperature sensitive plp2 alleles and of their suppressors is beginning to elucidate the essential functions of PLP2.

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