Evidence for multiple mechanisms for membrane binding and integration via carboxyl-terminal insertion sequences

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Subcellular localization of proteins with carboxyl-terminal insertion sequences involves targeting and membrane integration.
• Cytochrome b5 membrane integration is promiscuous, spontaneous, and independent of membrane proteins, relying mainly on targeting.

Purpose of the Study:

• To compare the membrane integration mechanisms of cytochrome b5 with three other proteins (Vamp1, polyomavirus middle-T antigen, Bcl-2) using carboxyl-terminal insertion sequences.
• To elucidate the factors governing selective membrane integration and identify distinct mechanisms.

Main Methods:

• Direct comparison of membrane integration mechanisms for cytochrome b5, Vamp1, polyomavirus middle-T antigen, and Bcl-2.
• Analysis of protein binding to membranes, including saturation, ATP-dependence, and sensitivity to trypsin treatment.

Main Results:

• Unlike cytochrome b5, Vamp1, polyomavirus middle-T antigen, and Bcl-2 exhibit membrane selectivity influenced by their integration mechanisms.
• Bcl-2 integration was inefficient in lipid vesicles. Vamp1 integration was saturable, ATP-dependent, and trypsin-sensitive.
• Polyomavirus middle-T antigen's insertion sequence mediated membrane binding but not bilayer integration.

Conclusions:

• At least two distinct mechanisms for membrane integration of proteins with insertion sequences exist: one primarily targeting-mediated and another relying on target membrane factors.
• Protein insertion sequence structure dictates the requirement for specific membrane-bound receptors for integration.
• Hydrophobicity alone is insufficient for insertion sequence-mediated membrane integration.