UNIQUE HEXOKINASE MESSENGER-RIBONUCLEIC-ACID LACKING THE PORIN-BINDING DOMAIN ARE DEVELOPMENTALLY EXPRESSED IN MOUSE SPERMATOGENIC CELLS

We have identified cDNAs representing three hexokinase mRNAs (Hk1-sa, Hk1-sb, Hk1-sc) by screening mouse spermatogenic cell cDNA libraries w ith a mouse hepatoma cell line hexokinase (Hk1) cDNA [Arora KY, Fanciu lli M, Pederson PL. J Biol Chem 1990; 265:6481-6488]. Although all thr ee cDNAs show 99% identity to the somatic Hk1 cDNA sequence throughout most of their coding region, they differ from this sequence at the 5' end. They contain a common spermatogenic cell-specific sequence and a sequence unique to each cDNA immediately 5' to the common domain. How ever, they lack the porin-binding domain (PBD) present in this region of Hk1, used for binding to a pore-forming protein in the outer mitoch ondrial membrane. These observations appear to support a model propose d by others for hexokinase gene evolution in mammals. In addition, we found that Hk1-sb has an internal sequence that is not present in Hk1, Hk1-sa, or Hk1-sc. Moreover, Hk1-sa and Hk1-sb transcripts are develo pmentally expressed in mouse spermatogenic cells. Hk1-sa mRNA is first expressed during meiosis and continues to be present in postmeiotic g erm cells, while the more abundant Hk1-sb mRNA is detected only in pos tmeiotic germ cells. These and other findings suggest that enzymes enc oded by Hk1-sa, Hk1-sb, and Hk1-sc are present only in spermatogenic c ells.