Cloning and characterization of cDNAs encoding steroidogenic acute regulatory protein from freshwater stingrays (Potamotrygon spp.).

The steroidogenic acute regulatory protein (StAR) is critical to the regulated synthesis of steroids in vertebrates. We have isolated cDNA sequences encoding StAR in the freshwater stingrays Potamotrygon hystrix and P. motoro. A single P. hystrix StAR transcript (3376 bp) and two overlapping P. motoro StAR transcripts (1272 and 3365 bp) were isolated. The P. hystrix and P. motoro StAR transcripts contain open reading frames encoding proteins of 284 amino acids that are 99% identical to each other and 56-64% identical to other StAR proteins. Pregnenolone synthesis by green monkey kidney (COS-1) cells transfected with an expression construct encoding a human cholesterol side chain cleavage/adrenodoxin reductase/adrenodoxin fusion protein was increased 16-fold by coexpression with a pCMV5/P. motoro StAR expression construct. Northern blot analysis revealed a single 4000 bp StAR transcript in the P. motoro interrenal gland, but RT-PCR indicates StAR mRNA is also expressed in the brain, gonads, atria, ventricle, gill (female only) and muscle (female only). Expression in extragonadal and extraadrenocortical tissues is an indication that StAR may be critical to processes other than steroidogenesis. The longest P. motoro StAR transcript contains a sequence with great similarity to short interspersed repetitive elements found in other elasmobranchs. This study is the first to isolate and characterize elasmobranch StAR cDNA sequences and to demonstrate the activity of a nonmammalian StAR protein in a heterologous expression system.