Expression of full-length and truncated Fyn tyrosine kinase transcripts and encoded proteins during spermatogenesis and localization during acrosome biogenesis and fertilization

We report that full‐length and truncated transcripts of Fyn tyrosine protein kinase are expressed during testicular development. Truncated Fyn (tr‐Fyn) transcripts encode a 24 kDa protein with a N‐terminal (NT) domain, a complete Src homology (SH) 3 domain and an incomplete SH2 domain. The kinase domain is missing in tr‐Fyn. In contrast, full‐length Fyn transcripts encode a 59–55 kDa protein. Fractionated spermatids by centrifugal elutriation express tr‐Fyn transcripts and protein, but not full‐length Fyn transcripts and protein. Neither full‐length Fyn nor tr‐Fyn transcripts and encoded proteins are detected in elutriated pachytene spermatocytes. Sertoli cells express full‐length and truncated Fyn throughout testicular development. In contrast, sperm contain full‐length Fyn transcripts and protein but not the truncated form. tr‐Fyn protein is visualized at the cytosolic side of Golgi membranes, derived proacrosomal vesicles, along the outer acrosome membrane and the inner acrosome membrane–acroplaxome complex anchoring the acrosome to the spermatid nuclear envelope. Fyn and phosphotyrosine immunoreactivity coexist in the tail of capacitated sperm. During fertilization, the Fyn‐containing acroplaxome seen in the egg‐bound and egg‐fused sperm is no longer detected upon decondensation of the sperm nucleus. tr‐Fyn expands the catalog of truncated tyrosine protein kinases expressed during spermiogenesis. We suggest that the NT and SH3 domains of tr‐Fyn may recruit adaptor and effector proteins, in particular GTPase activating proteins, required for acrosome–acroplaxome biogenesis, acroplaxome F‐actin dynamics and Sertoli cell function. During fertilization, full‐length Fyn in the acroplaxome may contribute to a transient local signaling burst during the early events of sperm–egg interaction. Mol. Reprod. Dev. 76: 832–843, 2009. © 2009 Wiley‐Liss, Inc.