Significance of hydrophobic and charged sequence similarities in sodium-bile acid cotransporter and vitamin D-binding protein macrophage activating factor

Ibne Raihan Zunaid; Stefania Pacini; Marco Ruggiero

AN only after 4 h incubation, reached a peak at 48 h, and returned below baseline values at 120 h. PMF, diluted 1:10 with PBS, had an initial value (at time 0) higher than that observed with PBS alone and higher than that observed with purified DBP-MAF; we interpret this result as indication that PMF has a much higher NAGAB_HUMAN-binding activity in comparison with purified DBP-MAF. At every time point, the PMF showed significantly higher activity in comparison with purified DBP-MAF. At 120 h, the activity of PMF was still well above the value obtained with the negative control, PBS, whereas DBP-MAF did not show any residual activity (not shown). Other experiments performed with 1:100 dilution showed a similar trend and the activity of PMF was always higher than that of purified DBP-MAF even at the highest dilution. These results led us . CC-BY-NC-ND 4.0 International license author/funder. It is made available under a The copyright holder for this preprint (which was not peer-reviewed) is the . https://doi.org/10.1101/2020.03.03.975524 doi: bioRxiv preprint to conclude that PMF has a DBP-MAF activity more than 100 fold higher than that of purified DBP-MAF. We attribute this higher activity to the presence of vitamin D 3 and fatty acids that are natural constituents of milk and colostrum. The presence of these hydrophobic moieties may favor interaction between DBP-MAF and NAGAB_HUMAN stabilizing the complex formed by the two proteins. The sequence similarities shown in Fig. 3 seem to support this hypothesis that is further corroborated by the recent observation that DBP-MAF conjugated with vitamin D 3 is much more efficient than DBP-MAF alone as an immune stimulating factor (15) . Hydrophobic interactions between the sequences of DBP-MAF and preS1 and domain IV of L protein may also help interpreting the biological effects reported in Fig. 4 (insert) . In this case, we can hypothesize two binding sites located at the two extremities of L protein. These interactions would be stabilized, as in the case of DBP-MAF/NAGAB_HUMAN complexes, by vitamin D 3 and fatty acids. This hypothesis is corroborated by the observation that viral proteins are able to bind directly vitamin D 3 (16) , and fatty acids show anti-viral properties that are due to prevention of entry of the viral genome into the host cell (17) as we propose being the case for the interaction between DBP-MAF/vitamin D 3 /fatty acids and preS1 and L protein hydrophobic sequences. In conclusion, based on the in silico and in vitro observation reported in this study, The copyright holder for this preprint (which was not peer-reviewed) is the . https://doi.org/10.1101/2020.03.03.975524 doi: bioRxiv preprint we propose that vitamin D 3 and fatty acids play a pivotal role in facilitating hydrophobic interactions between proteins showing stretches of similarities.