N-terminal domains and site-specific glycosylation regulate the secretion of avian melanocortin inverse agonists, agouti signaling protein (ASIP) and agouti-related protein (AGRP).

Agouti signaling protein (ASIP) and agouti-related protein (AGRP) are paralogous inverse agonists of melanocortin receptors with distinct physiological roles, but their structural and biochemical properties in birds remain poorly understood. Here, we characterized chicken ASIP and AGRP proteins. Analysis of available sequences revealed that a motif resembling the mammalian proprotein convertase 1/3 (PC1/3, also known as PCSK1) cleavage site is conserved across a broad range of avian orders, but Western blot analysis of transfected Chinese hamster ovary (CHO-K1) cells and chicken hypothalamus detected no cleavage, suggesting that avian AGRP may not be post-translationally processed at this site. Chicken ASIP mRNA contains an in-frame upstream ATG (uATG) and a putative N-linked glycosylation site at Asn-42, both conserved across multiple avian orders. Overexpression in CHO-K1 cells showed that ASIP translated from either ATG produces a mature protein of the same size that is N-glycosylated at Asn-42 and exhibits markedly lower secretion efficiency than AGRP. Domain-swapping experiments revealed that the N-terminal domain reduces secretion, whereas a naturally occurring ASIP-b variant with an additional N-glycan at Asn-47 shows enhanced secretion. Proteasome inhibition increased intracellular ASIP, and endoglycosidase H (Endo H) sensitivity indicated endoplasmic reticulum (ER) retention, suggesting that the N-terminal domain limits secretion via ER-associated proteasomal degradation. These findings reveal species-specific post-translational regulation of avian melanocortin inverse agonists, in which N-terminal features and site-specific N-glycosylation determine secretion efficiency, likely contributing to their distinct roles in pigmentation and hypothalamic energy balance.