The Albumins of Chinook Salmon (Oncorhynchus tshawytscha) and Brown Trout (Salmo trutta) Appear to Lack a Propeptide

Plasma samples from two members of the Salmonidae family, the chinook salmon (Oncorhynchus tshawytscha) and brown trout (Salmo trutta), were examined. Albumin, initially identified as the predominant anionic palmitate-binding band on agarose gel electrophoresis of plasma, was purified from both species by DEAE-ion exchange chromatography. Albumin has a plasma concentration of approximately 15 mg/ml in both species. Like other fish species, neither trout nor salmon albumin binds nickel; a characteristic of many mammalian albumins. Salmon and trout albumins have molecular masses of 65 and 67 kDa, respectively, indicating some sequence differences. However, N-terminal sequencing of the first 15 residues of both these proteins indicated identical sequences of1QNQICTIFTEAKEDG15−. This showed that the mature N-terminal sequence (SQAQNQICTIFTEAKEDG−) predicted from the cDNA of Atlantic salmon albumin is in fact incorrect, with the actual N-terminus for salmonids starting three amino acids later than that predicted. Examination of the Atlantic salmon cDNA sequence suggested that salmonid albumin is unique, in lacking a propeptide. No proalbumin convertase site (RXYR/XYRR) is present and it appears that the salmonid albumin precursor is cleaved only by the signal peptidase, between −1 Ala and +1 Gln, to produce mature albumin. This site has a preferrred motif for the signal peptidase of −1 Ala and −3 Ser, as well as −2 Gln. Thus, salmonid albumin possesses a 21-residue prepeptide, but no propeptide.