Analysis of the primary structure predicts that CTRP3 is a highly hydrophilic secreted protein, with an N-terminal hydrophobic signal peptide, and no transmembrane domains (43). Experimental work confirms that CTRP3 is a secreted protein and circulates in the blood, which indicates that the physiological function of CTRP3 occurs through endocrine mechanisms (51, 75). Additionally, CTRP3 has a series of N-terminal Collagenous repeats (Gly-X-Y), and a highly conserved C-terminal globular domain (76), thus placing CTRP3 within the expanding C1q TNF Superfamily (62) (Fig. 1). CTRP3 shares sequence homology with adiponectin (38% in mouse and 36% in human), and is highly conserved (95.9% identity between human and mouse proteins) (36). Additionally, there are two splice variants of CTRP3 that have been identified. The longer splice variant, designated as CTRP3B, encodes an extra 73 N-terminal amino acids due to the retention of intron 1. CTRP3B contains a highly conserved N-linked glycosylation site that is not present on the original splice variant, CTRP3A (51). At this time the functional significance of the splice variants of CTRP3 are unknown as research has focused almost exclusively
