SEQUENCES OF INTEREST: Homologous Sequences in Steroidogenic Enzymes, Steroid Receptors and a Steroid Binding Protein Suggest a Consensus Steroid-Binding Sequence

Abstract

The amino acid sequences of two steroidogenic enzymes, P450c17 (steroid 17 alpha-hydroxylase/17,20 lyse) and P450c21 (steroid 21-hydroxylase), are only 28.9% identical. However, these proteins share a region of 21 amino acids bearing 17 identical residues, which we previously suggested may represent the steroid binding site. We assembled a sequence database of known steroid-binding proteins and searched this with the sequence of this 21 amino acid region. The steroidogenic enzymes, P450c17, P450c21, P450scc (the cholesterol side-chain cleavage enzyme), and P450c11 (steroid 11 beta/18-hydroxylase) share a subregion of 17 amino acids having at least 15 identical residues. Related sequences were identified in a computerized search of the available sequences of steroid hormone receptors and binding proteins. These sequences were invariably found within larger domains previously associated with steroid binding. From these we propose a more general consensus sequence of LPLLL +/- 000KDRE0LKRL +/- PV, where +/- refers to any charged amino acid, and 0 refers to an uncharged amino acid. This consensus sequence predicts 147 or 187 total amino acids in 11 human proteins examined (78.6%). An equivalent degree of sequence identity, 178 of 221 amino acids (80.5%) was found among 13 animal homologs of these human proteins. The ability of this consensus sequence to predict 325 of 408 amino acids (79.7%) strongly suggests this sequence is necessary, if not sufficient, for a steroid binding site in many proteins. Lecithin-cholesterol acetyl transferase, cholesterol ester transfer protein, and steroid sulfatase did not have sequences similar to our consensus sequence.

Extraadrenal steroid 21-hydroxylation is not mediated by P450c21.

“…As the rat is the most widely used experimental animal for studying steroidogenesis, we wished to determine if the rat expressed P450c21 mRNA outside the adrenal. Rat P450c21 cDNA has not been cloned, but our demonstration of a highly conserved putative steroid-binding site, in addition to the known species conservation of the heme-binding site in steroidogenic P450s (20), suggested that some regions of the rat and human P450c21 sequences would be sufficiently similar (i.e., nearly identical) to yield hybrids resistant to RNase A. In the absence of a known rat P450c21 sequence, the number, length, and location of the regions of high similarity between rat and human P450c21 could not be determined.…”

Transcript encoded on the opposite strand of the human steroid 21-hydroxylase/complement component C4 gene locus.

“…These include a gene on the opposite strand of the gene for rat gonadotropin-releasing hormone (23), two Drosophila loci (24,25), a mouse locus of unknown function (26), and a newly described member of the c-erbA gene family on the opposite strand from the gene encoding a rat (27) and human (28) and the transcript from its opposite strand is unknown, it is obviously of interest that both mRNAs are found in the adrenal, suggesting a role in steroidogenesis. Potential roles in cholesterol transport or as a trans-acting regulatory protein cannot be evaluated at present, but the opposite-strand transcript we have described probably cannot encode another steroidogenic enzyme, as the genes for these are largely known (2), the available protein sequence lacks a steroid binding site (29), and only deficiency of 21-hydroxylase is linked to HLA (1, 2). P450c21 and the mRNA encoding the 2.7-kb cDNA are both found in the adrenal, but we do not know whether they are expressed in the same cells.…”

Cloning of a Syrian hamster cDNA related to sexual dimorphism: establishment of a new family of proteins

“…In this sense, the steroidbinding ability of PSBP, UG/CC10 and msABP [9,10,12,35] might be relevant, if it is demonstrated for the FHG22 protein. However, no homologies have been found to the canonical (but not general) steroid-binding sequence motif [42] in any of these proteins, while it has been found that besides steroids and polychlorinated biphenyls, rabbit UG binds retinoids and probably membrane proteins [21,43] and human CC10 binds calcium [20]. The protein might be secreted both to the saliva and eye secretion and bind different ligands, playing a role similar to the immunomodulatory/protective action described for UG/CC10 in the female genital tract [10,44] and the lungs [31,32].…”