Vitamin K-dependent proteins have been shown to play a major role in vertebrates in blood coagulation and its regulation, as well as in bone resorption and tissue mineralization. The vitamin K-dependent proteins contain gamma carboxyglutamic acid residues that are generated in a post-translation event within the lumen of the endoplasmic reticulum. This reaction is catalyzed by a gamma glutamyl carboxylase that is widely distributed in numerous vertebrate and non-vertebrate tissues. This investigation will focus on gaining insight into the biological role of four novel vitamin K-dependent proteins that are single transmembrane proteins. These proteins range in size from 17 kDa to 25 kDa and contain from nine to thirteen gamma- carboxyglutamic acid residues. They have been called PRGP1, PRGP2, TMG3, and TMG4. Genes for two of these transmembrane proteins, PRGP1 and TMG3 are located on the X chromosome, while genes for PRGP2 and TMG4 are found on chromosome 19 and 11, respectively. Studies with transfected mammalian cells have now shown that they require vitamin K for their post-translational modification in a reaction that is inhibited by warfarin. The carboxyl region of the four transmembrane proteins bind to Nedd4-1ike proteins, suggesting that they may play a role in the ubiquitin cycle. Nedd4-1ike proteins have a C2 domain, two to four WW domains, and a HECT domain that functions as an E3 ubiquitin ligase in the ubiquitin cycle. The mechanism and specificity of these reactions will be examined in detail in these investigations.