Comparative proximity biotinylation implicates the small GTPase RAB18 in sterol mobilization and biosynthesis

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Warburg Micro syndrome is an autosomal recessive condition caused by loss of functional RAB18.
• Understanding RAB18's cellular functions is crucial for elucidating the disease's mechanisms.

Purpose of the Study:

• To identify RAB18 effectors using proximity biotinylation.
• To investigate a potential role for RAB18 in cholesterol biosynthesis.

Main Methods:

• Proximity biotinylation was employed to map RAB18 interactions.
• Cellular assays were performed on RAB18-null and RAB3GAP1-null cells.
• Cholesterol biosynthesis pathway intermediates were analyzed.

Main Results:

• A set of 28 RAB18 interactions dependent on the RAB3GAP1-RAB3GAP2 guanine nucleotide exchange factor complex was identified.
• Novel interactions with SEC22A, TMCO4, and INPP5B were validated.
• Accumulation of the cholesterol precursor lathosterol and impaired de novo cholesterol biosynthesis were observed in cells lacking functional RAB18 or ORP2.

Conclusions:

• RAB18 interacts with proteins involved in membrane remodeling and lipid transport, including sterol-related proteins.
• The findings suggest that Micro syndrome may be a cholesterol biosynthesis disorder.
• Proximity biotinylation is effective for interrogating guanine nucleotide exchange factor-dependent Rab interactions.