The pacific beetle cockroach, Diploptera punctata, is a viviparous cockroach that produces a milk-like substance to support the growing embryo with a brood sac. The structure of the in vivo grown crystals present in the gut of the embryo showed that the milk-derived crystals are heterogenous and are made of three proteins (called Lili-Mips). Multiple fatty acids could be modeled into the active site, and we hypothesized that each of the three isoforms of the protein bound to a different fatty acid. We previously reported that the recombinantly expressed Lili-Mip2 has a structure similar to the structure of the protein determined from in vivo crystals, and this single isoform also binds to several fatty acids. In this study, we aimed to probe the specificity and affinity of fatty acid binding and test the stability of different isoforms. We show that all the isoforms can bind to different fatty acids with very similar affinities, and the local abundance of a fatty acid determined bound fatty acid ratios. Lili-Mips' thermostability is pH dependent, where stability is highest at acidic pH and declines as the pH increases to physiological levels near 7.0. The measurement of the pH in the gut lumen and the gut cells suggests that the pH in the gut is acidic and the pH inside the gut cells is closer to neutral pH. We propose that the protein has evolved to be highly stable in the acidic gut lumen and, when absorbed inside the gut cells, becomes less stable to enable the breakdown of the glycosylated lipo-protein complex to provide essential metabolites for survival and development of the embryo. The different orientations of Phe-98 and Phe-100 control the binding pocket volume and allow the binding of different chain-length fatty acids to bind with similar affinities.
Nucleotide Sugar Transporters (NSTs) belong to the SLC35 family (human solute carrier) of membrane transport proteins and are crucial components of the glycosylation machinery. NSTs are localized in the ER and Golgi apparatus membranes, where they accumulate nucleotide sugars from the cytosol for subsequent polysaccharide biosynthesis. Loss of NST function impacts the glycosylation of cell surface molecules. Mutations in NSTs cause several developmental disorders, immune disorders, and increased susceptibility to infection. Atomic resolution structures of three NSTs have provided a blueprint for a detailed molecular interpretation of their biochemical properties. In this work, we have identified, cloned, and expressed 18 members of the SLC35 family from various eukaryotic organisms in Saccharomyces cerevisiae. Out of 18 clones, we determined Vrg4 from Chaetomium thermophilum (CtVrg4) is a GDP-mannose transporter with an enhanced melting point temperature (Tm) of 56.9 °C, which increases with the addition of substrates, GMP and GDP-mannose. In addition, we report—for the first time—that the CtVrg4 shows an affinity to bind to phosphatidylinositol lipids.
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