Characterization of Rat Caltriin Structure and its interactions with model membranes at the air-water interface using molecular dynamics simulations and other bioinformatics tools

Abstract

Rat caltrin (calcium transport inhibitor), the small and basic protein of the seminal plasma binds to the spermatozoa during ejaculation and inhibits sperm extracellular Ca2+ uptake. Thus, it prevents the sperm spontaneous acrosomal exocytosis along the female reproductive tract. Although the sequence and some biological features of rat caltrin were studied, its physicochemical properties and 3D structure are still unknown. In this work we predicted the rat caltrin 3D structure, by molecular homology modeling and threading, which maintained its secondary and tertiary structures along molecular dynamics simulations. The molecular structure was further characterized by circular dichroism. Surface electrostatic potentials and electric fields were calculated using the Poisson-Boltzmann equation and the overall protein dipole was also evaluated. Bioinformatics tools and available web servers were used to deeply analyze physicochemical characteristics such as Kyte and Doolittle Hydropathy score, solvent accessibility, Wimley-White whole-residue hydrophobicity and helical wheel projections. The equilibrium spreading pressure was estimated by Gibbs adsorption isotherms. Interactions between rat caltrin and phospholipids model membranes were defined by penetration (cut off) studies. Rat caltrin was able to penetrate into the membranes, mainly in negatively charged surfaces and expanded lateral phase states, and the amino acid residues involved in the protein-membrane interaction were also predicted. To further characterize the protein binding to membrane surfaces we carried out simulations in the presence of negatively charged bilayers. Results presented have significant relevance to understanding the molecular mechanisms of caltrin to modulate physiological processes associated with fertilization.