Does the Golgi Reassembly and Stacking Protein (GRASP) Behave as a Well-Structured Protein in Solution?

Abstract

Among all proteins localized in the Golgi apparatus, a two-PDZ domain protein plays an important role in the assembly of the cisternaes and is also responsible for many other functions. This class of proteins, known as Golgi Reassembly and stacking Proteins (GRASP), has puzzled many researchers due to its large array of functions. In this work, the GRASP homologue in the fungus Cryptococcus neoformans (CnGRASP) was studied. This protein is associated with the unconventional secretion mechanisms required for the export of the most important virulence factor in that fungus. Biophysical techniques were used to assess structural aspects of CnGRASP in solution. We were able to detect a relevant secondary structural content, but with a large amount of disordered regions. The overall structure is less compacted compared to those values expected for a globular protein, which also leads to a high structural flexibility and water accessibility of the hydrophobic core. All the results together indicate an unusual behavior of CnGRASP in solution that closely resembles the behavior previously observed for molten globule proteins. To the best of our knowledge this is the first direct observation of the molten globule-like behavior of a GRASP protein in physiological conditions. We also speculate the possible implications due to this unusual behavior and how it can explain the multiple facets associated with this intriguing class of proteins. Financial support: FAPESP, CNPq, CAPES.