Mutations in the Rab33b protein that lead to the skeletal disease Smith-McCort dysplasia result in unstable proteins...

The human skeletal disease Smith McCort dysplasia is known to be caused by mutations in the RAB33B gene. Despite there being detailed genetic and medical studies about the patients carrying these mutated genes, there is a paucity of information about these mutations at the molecular and cellular level. The RAB33B gene encodes the small GTP binding protein Rab33b, which primarily localises to the Golgi apparatus in cells, and plays roles in membrane traffic and autophagy. In recent years, several different mutations in the RAB33B gene have been reported, potentially giving rise to both prematurely truncated proteins and also proteins containing single amino acid substitutions. Importantly, no work to date has examined the consequences of expression of these Rab33b variants in cells. In the study presented here we use a model cell culture system to seek to understand what the consequences might be to cells expressing five of the reported disease-causing Rab33b variants. We specifically examine the ectopic expression of two truncated and three single amino acid substitution variants in cultured cells. Our results reveal that all of these mutants show subcellular mislocalisation and fail to accumulate on Golgi membranes. We also demonstrate that each of these mutants are unstable and suffer from premature degradation in cells. Finally, overexpression of the single amino acid substitution variants in cells induced for autophagy causes a severe reduction in the number of autophagosomes as defined by the number of LC3B-positive puncta. Our results provide the first molecular insight into the cellular effects caused by five of the reported Rab33b mutants that give rise to Smith McCort dysplasia.