Molecular cloning and characterization of a novel dual-specificity proteinphosphatase possibly involved in spermatogenesis

Dual-specificity protein phosphatases (DSPs) play roles in the regulation o f mitogenic signal transduction for extracellular stimulation and the cell cycle. In the present study, we identified a novel DSP, termed TMDP (testis - and skeletal-muscle-specific DSP). Nucleotide sequence analysis of TMDP c DNA indicated that the open reading frame of 597 bp encodes a protein of 19 8 amino acid residues with a predicted molecular mass of 22.5 kDa. The dedu ced amino acid sequence contains a motif for a conserved catalytic domain o f DSPs and shows highest similarity to human Vaccinia HI-related phosphatas e (45.5 % identity) but low homology to the mitogen-activated protein kinas e phosphatase and CDC25 subfamilies of DSPs. Recombinant TMDP protein exhib ited intrinsic phosphatase activity towards both phospho-seryl/threonyl and -tyrosyl residues of myelin basic protein, with similar specific activitie s in vitro. Northern-blot analysis revealed that TMDP is most abundantly ex pressed in the testis. The expression in the testis is characterized as fol lows: (i) TMDP mRNA first appeared 3 weeks after birth, corresponding to th e time that meiosis begins; (ii) TMDP mRNA was abundant in fractionated spe rmatocytes and round spermatids; and (iii) hybridization in situ showed tha t the TMDP mRNA is localized in spermatocytes and/or spermatids in seminife rous tubules. These data demonstrate that TMDP is a novel DSP abundantly ex pressed in the testis and suggest that TMDP may be involved in the regulati on of meiosis and/or differentiation of testicular germ cells during sperma togenesis.