Mohammad A.M. Basalamah, Ph.D.

Dissertation research performed under the guidence of Dr. Cora-Jean Edgell

ABSTRACT
    Testican is a selectively expressed, highly conserved gene product of unknown function. We have previously shown that testican is expressed by human endothelial cells in culture, and by endothelial cells as well as certain other cell types in vivo. Testican mRNA encodes a signal-like sequence at the N-terminal suggesting an extracellular protein. Therefore, we sought to determine whether testican protein is present in blood. We have developed testican-specific antibodies and used them to identify the gene product in western blots of blood proteins. The apparent molecular weight of testican in plasma is much larger than that calculated for the encoded polypeptide, suggesting glycosylation or other post-translational covalent modifications. The large form is unstable in plasma and converted to 3 smaller stable forms by separable plasma factors. Using a recombinant testican representing the whole open reading frame expressed by cultured human cells, we found that the protein is primarily in the culture fluid, and we determined its signal sequence cleavage site by amino acid sequencing. A portion of the recombinant protein, similar to the plasma form, is much larger than what can be ascribed to the encoded sequence alone. We present evidence that the large form of the recombinant testican is a chondroitin sulfate proteoglycan. Sequence based structural features are described, including a domain associated with thyropin-type cysteine protease-inhibitors. Thus, testican may have a function related to protease inhibition in the blood, and the size conversion we have observed may be important in its functional activation or decay.