Microvillus inclusion disease-causing MYO5B point mutations exert differential effects on motor function

Microvillus inclusion disease (MVID) is a rare congenital diarrheal disorder typically caused by loss of function mutations in the unconventional myosin, myosin 5b (MYO5B), which leads to the mistrafficking of apical components in enterocytes. MVID can manifest in two phenotypes: in both the intestine and liver or the liver alone. Although previous studies seeking to understand MVID disease pathology used MYO5B KO models, many patients have point mutations and thus express a dysfunctional MYO5B. How these point mutations lead to a broad spectrum of disease severity and the development of two distinct disease phenotypes is still not known. Here, we investigate the effect of MVID patient mutations on the function of the MYO5B motor domain, independent of cargo binding, using confocal imaging and fluorescence recovery after photobleaching. Patient mutations demonstrated a range of effects in these assays, from rigor-like behavior to loss of actin binding. Additionally, analysis of fluorescence recovery after photobleaching turnover kinetics suggests that some mutations negatively impact the ability of MYO5B to stay bound to actin. Collectively, our findings indicate that patient mutations affect the MYO5B motor domain in diverse ways, consistent with the spectrum of phenotypes observed in patients.