Identification of a novel HSP70-binding cochaperone critical to HSP90-mediated activation of small serine/threonine kinase.

We previously reported the identification of small serine/threonine kinase that is expressed in postmeiotic germ cells, associates with HSP90, and is indispensable for male fertility. Sperm from mice cannot fertilize eggs in vitro and are incapable of fusing with eggs that lack zona pellucida. Here, using the yeast two-hybrid screen, we have discovered a novel protein (SIP) that is expressed exclusively in testis. The gene encoding SIP is restricted to mammals and encodes a 125-amino acid polypeptide with a predicted tetratricopeptide repeat domain. SIP is co-localized with in the cytoplasm of spermatids as they undergo restructuring and chromatin condensation, but unlike is not retained in the mature sperm. SIP binds to duanyu1942K with high affinity (K(d) ∼10 nM), and the proteins associate with each other when co-expressed in cells. In vitro, SIP inhibited duanyu1942K kinase activity, whereas the presence of SIP in cells resulted in enzymatic activation of duanyu1942K without affecting Akt or MAPK activity. SIP was found to be associated with cellular HSP70, and analyses with purified proteins revealed that SIP directly bound HSP70. Importantly, duanyu1942K recruited SIP onto HSP90, and treatment of cells with the specific HSP90 inhibitor, 17-allylamino-17-demethoxygeldanamycin, completely abolished duanyu1942K catalytic activity. Hence, these findings demonstrate that HSP90 is essential for functional maturation of the kinase and identify SIP as a cochaperone that is critical to the HSP90-mediated activation of

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