%0 Journal Article
%T A Novel Membrane Protein-Specific Serine/Threonine Kinase: Tissue Distribution and Role in Sperm Maturation
%A Debjani Nath
%A Arpita Bhoumik
%A Sujoy Das
%A Debdas Bhattacharyya
%A Sandhya R. Dungdung
%A Gopal C. Majumder
%J ISRN Urology
%D 2012
%R 10.5402/2012/789105
%X Our recent studies have described for the first time the purification of an ectoprotein kinase to apparent homogeneity using caprine sperm as the model. Purified ectokinase (CIK) is a novel membrane protein-specific kinase that phosphorylates serine and threonine residues of ectophosphoproteins. This study, using ELISA based on ecto-CIK antibody demonstrates that ecto-CIK level is remarkably higher in the sperm membrane than in the cytosol. The epididymal sperm maturational event as well as sperm vertical velocity is associated with a significant increase in the ecto-CIK level. Ecto-CIK, the membrane protein-specific kinase, is also present in all the tissues tested and is predominantly localized in the cell membrane. Ubiquitous localization of the novel kinase on the mammalian cell membrane suggests that the kinase may play pivotal role in gamete as well as somatic cell regulation by modulating membrane biology through serine/threonine phosphorylation of specific membrane proteins located in the ectodomains. 1. Introduction Testicular spermatozoa are immotile and undergo maturation during transit through the caput, corpus, and cauda parts of the epididymis where they acquire forward motility and fertility potential. Studies from our laboratory have provided several lines of evidence for the occurrence of a cyclic AMP-independent protein kinase (ecto-CIK) on the external surface of caprine (Capra indicus) cauda epididymal mature spermatozoa that causes phosphorylation of the membrane-bound phosphoproteins [1¨C3]. Our recent studies using caprine sperm as the model have described for the first time the purification to apparent homogeneity of an ectoprotein kinase [4] as well as its major phosphoprotein substrate: MPS [5] located on the sperm external surface. Ecto-CIK is a 115£¿kDa protein made up of two subunits: 63 and 55£¿kDa. It is a strongly basic protein. It is also strongly immunogenic. The ectokinase has nearly 30 times greater affinity for MPS as compared to casein¡ªthe most potent exogenous protein substrate. Thereby, it demonstrates that CIK is a unique membrane protein-specific kinase, which specializes for phosphorylating the serine and threonine residues of the sperm outer cell-surface phosphoproteins [5]. MPS is a 100£¿kDa phosphoprotein. Both CIK and MPS are localized primarily in the acrosomal cap area of the external surface of the mature sperm head as demonstrated by indirect immunofluorescence studies [4, 6]. We have demonstrated that ecto-CIK through its substrate protein (MPS) plays a vital role in the regulation of sperm forward
%U http://www.hindawi.com/journals/isrn.urology/2012/789105/