Biophysical characterisation of a Plasmodium Falciparum GRASP protein

Plasmodium falciparum is a species of unicelular eukaryotic parasites, responsible for malaria, that infect human liver and red blood cells. P. falciparum resides within a parasitophorous vacuolar membrane when infecting erythrocytes, this membrane and the host cytosol are extensively modified during infection by secreted proteins in order to enhance parasite survival and pathogenicity. Golgi Reassembly and Stacking Proteins (GRASP) are a family of membrane associated proteins first identified in mammals, but present in all eukaryotes with the notable exception of plants. As per their name, GRASPs were first thought to be responsible for the stacked structure of golgi organelles. However, subsequent research has questioned their role in golgi organization and identified their participation in golgi remodeling during mitosis and unconventional protein secretion. N. Struck et al identified two distinct GRASP analogues in P falciparum. These two analogues are splice isoforms of the same gene and virtually identical with the exception of their N-terminal membrane anchoring motiffs, with PfGRASP1 having a mammal-like myristoylation site while PfGRASP2 has a yeast-like amphipathic helix. The presence of two analogues is unusual both because of the different anchoring types and because most lower eukaryotes do not possess more than one GRASP protein. This peculiarity could be explained by the need for differential localization and function of GRASPs in P. falciparum`s complex secretion machinery. 

In order to elucidate those questions, we have expressed PfGRASP1 in E. coli cells and probed the purified protein structure with spectroscopic and optical methods such as circular dichroism, revealing a protein with partial intrinsically disordered character and an unexpected resistance to thermal denaturation. Further research into the structural characteristics of PfGRASP 1 and 2 should serve as a springboard from which to investigate their curious duplicity and functional significance.