Involvement of Nucleophosmin (NPM1/B23) in Assembly of Infectious HPV16 Capsids

Area of Science:

• Virology
• Molecular Biology
• Cell Biology

Background:

• The Human papillomavirus type 16 (HPV16) capsid is assembled from major (L1) and minor (L2) structural proteins.
• Nucleophosmin (NPM1), also known as B23, is a host nucleolar protein involved in various cellular processes.

Purpose of the Study:

• To investigate the interaction between HPV16 minor capsid protein L2 and host nucleolar protein NPM1 during HPV16 pseudovirus (PsV) assembly.
• To determine the role of NPM1 in the stability and infectivity of HPV16 PsV.

Main Methods:

• Co-immunoprecipitation assays to detect protein interactions.
• Confocal microscopy to assess protein colocalization within cells.
• Production of HPV16 PsV in cell lines with varying NPM1 levels and subsequent infectivity assays.
• Analysis of PsV trypsin sensitivity and L2 protein levels during endocytosis.

Main Results:

• HPV16 L2 protein interacts with and colocalizes with NPM1 in the nucleus.
• Co-expression of L1 protein disrupts L2-NPM1 interaction and association with the nuclear matrix.
• HPV16 PsV produced in NPM1-deficient cells exhibit significantly lower infectivity, increased trypsin sensitivity, and premature L2 loss during endocytosis.
• NPM1 does not play a role in HPV infectious entry.

Conclusions:

• NPM1 plays a critical role in mediating correct L1-L2 interactions during HPV16 capsid assembly.
• Reduced NPM1 levels result in the formation of unstable HPV16 capsids, leading to premature L2 loss and impaired infectivity.
• NPM1 is essential for producing infectious HPV16 particles.