Repeat domain-associated O-glycans govern PMEL fibrillar sheet architecture

Area of Science:

• Cell Biology
• Biochemistry
• Structural Biology

Background:

• PMEL is a pigment cell-specific protein essential for melanosome biogenesis.
• It forms a functional amyloid matrix where the pigment melanin is deposited.
• The precise structural role of PMEL domains in matrix formation and function was unclear.

Purpose of the Study:

• To elucidate the structural role of the PMEL repeat (RPT) domain in amyloid matrix formation.
• To investigate the functional conservation and molecular determinants of RPT domain activity.
• To understand the contribution of O-glycosylation to PMEL amyloid morphology.

Main Methods:

• Electron tomography was used to analyze the melanosome matrix architecture.
• Comparative analysis of RPT domains across different vertebrate species.
• Functional assays and biochemical predictions to assess the role of O-glycosylation.

Main Results:

• The RPT domain, a proteolytic fragment, governs the sheet architecture of the PMEL amyloid matrix.
• The RPT domain is dispensable for amyloidogenesis but critical for shaping the matrix morphology for pigment adsorption.
• RPT domains are functionally conserved across vertebrates, with O-glycosylation identified as a key conserved feature essential for function.
• O-glycosylation, rather than amino acid sequence, is the primary determinant of the characteristic PMEL amyloid morphology.

Conclusions:

• The PMEL RPT domain acts as a crucial architectural modulator, optimizing the amyloid matrix for melanin deposition.
• Functional conservation of the RPT domain relies on O-glycosylation, highlighting its importance in structural biology.
• This study reveals O-glycosylation as a key post-translational modification driving the unique morphology of PMEL amyloid structures.