Archaebacterial and eukaryotic proteasomes prefer different sites in cleaving gonadotropin-releasing hormone

Thermoplasma 20 S proteasomes are composed of only two different types of subunits (designated as alpha and beta) but are nearly indistinguishable in their quaternary structure from eukaryotic 20 S proteasomes consisting of 14 distinct subunits. In this study, we compared both the nature and the rate of the proteolytic activities of Thermoplasma and of granulosa cell proteasomes on the neurohormone, gonadotropin-releasing hormone (GnRH), the degradation products of which can be unequivocally identified. Both Thermoplasma and granulosa proteasome degrade the decapeptide GnRH at the Trp3-Ser4, Ser4-Tyr5, Tyr5-Gly6, and Gly6-Leu7 bonds. While the main product of Thermoplasma proteasomes was a GnRH-(1-4) fragment, the main product of granulosa cell proteasome was a GnRH-(1-5) fragment, indicating that the principal degrading activity of Thermoplasma proteasome targets Ser4-Tyr5 bond, while the principal degrading activity of granulosa cell proteasome targets the Tyr5-Gly6 bond of GnRH. These differences in the degradation pattern of the neurohormone were observed when proteasome activities were compared both at 60 degrees C, the optimal temperature for Thermoplasma proteasomal activity, and at 37 degrees C, the optimal temperature of granulosa proteasome proteolytic activity. Although the catalytic mechanism is probably conserved from archaebacterial to eukaryotic proteasomes, our results suggest that there are striking differences in the preferred cleavage site of GnRH. This reflects the changes in the proteasomal subunit repertoire during evolution.