The amino acid transport system y+L/4F2hc is a heteromultimeric complex

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• The transmembrane protein 4F2hc is widely expressed in mammalian cells.
• Expression of 4F2hc in Xenopus laevis oocytes induces amino acid transport resembling system y+L.
• Previous studies suggested 4F2hc function depends on association with an endogenous protein.

Purpose of the Study:

• To investigate the role of an endogenous factor in 4F2hc-mediated amino acid transport.
• To determine if 4F2hc forms a heteromultimeric complex with another membrane protein.
• To identify the structural basis for system y+L amino acid transport activity.

Main Methods:

• Expressed human 4F2hc in Xenopus laevis oocytes.
• Utilized site-directed mutagenesis to alter cysteine residues (C109, C330) in 4F2hc.
• Assessed amino acid transport activity and sensitivity to mercury compounds (Hg2+, pCMB, pCMBS).

Main Results:

• System y+L-like transport activity saturated at low 4F2hc expression levels.
• Mutations in cysteine residue C109 partially reduced transport activity.
• Mercury compounds inactivated transport, indicating involvement of external cysteine residues, potentially linked via disulfide bonds.

Conclusions:

• 4F2hc requires intimate association with an endogenous oocyte membrane protein for system y+L amino acid transport.
• This study provides the first direct evidence for a heteromultimeric structure of an organic solute carrier in mammals.
• The findings elucidate the functional requirement of protein-protein interactions for solute transporter activity.