Biophysical characterization of intrinsically disordered human Golgi matrix protein GRASP65

Golgi Reassembly and Stacking Proteins (GRASPs), including GRASP65/GRASP55, were firstly found as stacking factors of Golgi cisternae. Their involvement in other processes, such as unconventional protein secretion (UPS), have been demonstrated, suggesting GRASPs act as interaction hubs. However, structural details governing GRASP functions are not understood thoroughly. Here, we explored the structural features of human cis-Golgi GRASP65 in aqueous solution and compared them with those from trans-Golgi GRASP55. Besides their distinct Golgi localization, GRASP65/55 also seem to be selectively recruited to mitosis-related events or to UPS. Despite preserving the monomeric form in solution seen for GRASP55, as inferred from our SEC-MALS and DLS data, GRASP65 exhibited higher intrinsic disorder and susceptibility to denaturant than GRASP55 (disorder prediction, urea denaturation and circular dichroism data). Moreover, spectroscopic and microscopic studies showed for GRASP65 the same temperature-dependent amorphous aggregation and time-dependent amyloid fibrillation at 37 °C seen for GRASP55. In the latter case, however, GRASP65 presented a lower aggregation rate than GRASP55. The present and previous data evidenced that intrinsic disorder and formation of higher-order oligomers, such as amyloid fibrils, are common features within GRASP family potentially impacting the protein's participation in cell processes.