Post-translational processing of the natural human thyrotropin receptor: Demonstration of more than two cleavage sites

Epitope-mapped monoclonal and polyclonal antibodies to the TSH receptor (TSHR) were used as immunoblot probes to detect and characterize the molecular species of the receptor present in normal human thyroid tissue. In reduced membrane fractions, both full-length (uncleaved) holoreceptor and cleavage-derived subunits of the holoreceptor were detected. Uncleaved holoreceptor species included a nonglycosylated form of apparent molecular mass 85 kDa and two glycosylated forms of approximately 110 and 120 kDa. The membranes also contained several forms of cleavage-derived TSHR-alpha and TSHR-beta subunits. TSHR-alpha subunits were detected by antibodies to epitopes localized within the amino terminal end of the TSHR ectodomain and migrated diffusely between 45-55 kDa, reflecting a differentially glycosylated status. TSHR-beta subunits were detected by antibodies to epitopes within the carboxyl end of the TSHR ectodomain. Several species of TSHR-beta subunit were present, the most abundant having apparent molecular masses of 50, 40, and 30 kDa. These data demonstrated that post-translational processing of the TSHR in human thyroid tissue involved multiple cleavage sites.