Regulated expression of p14 (cofactor A) during spermatogenesis.

Abstract

The correct folding of tubulins and the generation of functional alpha beta-tubulin heterodimers require the participation of a series of recently described molecular chaperones and CCT (or TRiC), the cytosolic chaperonin containing TCP-1. p14 (cofactor A) is a highly conserved protein that forms stable complexes with beta-tubulin which are not apparently indispensable along the in vitro beta-tubulin folding route. Consequently, the precise role of p14 is still unknown, though findings on Rb12p (its yeast homologue) suggest p14 might play a role in meiosis and/or perhaps to serve as an excess beta-tubulin reservoir in the cell. This paper investigates the in vivo possible role of p14 in testis where mitosis, meiosis, and intense microtubular remodeling processes occur. Our results confirm that p14 is more abundantly expressed in testis than in other adult mammalian tissues. Northern blot, Western blot, in situ hybridization, and immunocytochemical analyses have all demonstrated that p14 is progressively upregulated from the onset of meiosis through spermiogenesis, being more abundant in differentiating spermatids. The close correlation observed between the mRNA expression waves for p14 and testis specific tubulin isotypes beta 3 and alpha 3/7, together with the above results, suggest that p14 role in testis would presumably be associated to beta-tubulin processing rather than meiosis itself. Additional in vitro beta 3-tubulin synthesis experiments have shown that p14 plays a double role in beta-tubulin folding, enhancing the dimerization of newly synthesized beta-tubulin isotypes as well as capturing excess beta-tubulin monomers. The above evidence suggests that p14 is a chaperone required for the actual beta-tubulin folding process in vivo and storage of excess beta-tubulin in situations, such as in testis, where excessive microtubule remodeling could lead to a disruption of the alpha-beta balance. As seen for other chaperones, p14 could also serve as a route to lead excess beta-tubulin or replaced isotypes towards degradation.