Basolateral sorting in MDCK cells requires a distinct cytoplasmic domain determinant

Area of Science:

• Cell biology
• Molecular biology
• Membrane protein trafficking

Background:

• Basolateral transport of membrane proteins is crucial for cell polarity.
• Current models suggest basolateral sorting is a default pathway in some cell types.
• Cytoplasmic domain determinants are known to influence protein localization.

Purpose of the Study:

• To investigate the role of cytoplasmic domain determinants in basolateral transport of membrane proteins in MDCK cells.
• To determine if basolateral sorting signals are distinct from endocytosis signals.
• To challenge the default sorting model for basolateral membrane proteins.

Main Methods:

• Studying membrane protein transport in Madin-Darby canine kidney (MDCK) cells.
• Analyzing the role of specific cytoplasmic domain sequences, including tyrosine residues.
• Comparing transport pathways for different membrane proteins like Fc receptor, lysosomal glycoprotein Igp120, and LDL receptor.

Main Results:

• Basolateral transport in MDCK cells is not a default process but depends on cytoplasmic domain determinants.
• For some proteins (Fc receptor, Igp120), determinants resemble those for clathrin-coated pit endocytosis.
• For other proteins (LDL receptor), basolateral determinants differ from endocytosis signals, particularly tyrosine requirements.
• Eliminating a tyrosine in Igp120 blocked internalization and redirected it to the apical surface.
• Apical transport occurs for many proteins lacking specific basolateral determinants, suggesting widespread apical signals.

Conclusions:

• Basolateral transport in MDCK cells is an active, signal-dependent process.
• Cytoplasmic domain determinants for basolateral sorting are diverse and not universally linked to endocytosis signals.
• The presence of widely distributed apical transport signals explains why proteins reach the apical surface in the absence of basolateral determinants.