Relationship between platelet imidazoline receptor-binding peptides and candidate imidazoline-1 receptor, IRAS

A candidate human imidazoline-1 receptor, designated imidazoline receptor antisera-selected (IRAS) protein, was cloned based on immunoreactivity with antiserum against a purified imidazoline receptor binding peptide (IRBP antiserum). Human IRAS is 167 kD in size, different from 33- to 85-kD IRBP bands previously linked to the human platelet I(1) receptor. To explore the possible relationship between IRAS and these smaller proteins, seven different epitope-specific antisera against IRAS were raised in rabbits for comparison with IRBP antiserum. Focus was on antiserum(227-241), corresponding to amino acids No. 227 to 241 in IRAS, because this antiserum was found uniquely able to immunoprecipitate non-denatured 85-kD and 170-kD forms of IRAS from a human megakaryoblastoma cell line (MEG01), a model of platelet-producing cells. Human platelets lacked the 170-kD form of IRAS, but 33-kD and 85-kD bands were detectable and seemed to be possible fragments of full-length IRAS. The intensity of the 85-kD band detected by antiserum(227-241) was significantly correlated (r = 0.62, P = 0.04) with the intensity of the 33-kD band across 11 human platelet samples. A positive correlation between the intensities of the 33-kD and 85-kD bands is consistent with both being fragments of IRAS.