Viral structure and assembly: what is the role of matrix (M) proteins? In medical virology, which statement best describes where viral matrix proteins are located and what they primarily do in enveloped viruses?

Introduction / Context:
Matrix proteins (often abbreviated as M, MA, or tegument-like layers depending on the family) are key structural components of many enveloped viruses. This question tests your understanding of where matrix proteins are located and their central role during virion assembly and stability.

Given Data / Assumptions:

• Enveloped viruses possess a lipid bilayer derived from host membranes.
• Inside the envelope lies the nucleocapsid (genome plus capsid proteins).
• Matrix proteins frequently occupy the space between the envelope and the nucleocapsid.
• The options contrast surface spikes, naked virion composition, and nucleocore roles.

Concept / Approach:
Matrix proteins form a proteinaceous layer on the inner face of the viral envelope. They connect the cytoplasmic tails of envelope glycoproteins to the nucleocapsid. This scaffolding function stabilizes virion shape, orchestrates budding at host membranes, and can recruit host factors needed for assembly.

Step-by-Step Solution:
Identify matrix proteins as perenvelope scaffolds in enveloped viruses.Recall that spikes (for example, hemagglutinin, gp120) are transmembrane glycoproteins exposed externally—not matrix proteins.Recognize that naked viruses lack an envelope, so a matrix layer is not their defining feature.Select the choice stating matrix proteins anchor the envelope to the nucleocapsid.

Verification / Alternative check:
Examples include the M1 protein of influenza, the M protein of paramyxoviruses, and the MA (matrix) domain of retroviral Gag. All localize under the envelope and coordinate assembly and budding from host membranes.

Why Other Options Are Wrong:

• Surface spikes: those are distinct envelope glycoproteins, not matrix proteins.
• Mainly on naked viruses: incorrect; matrix associates with envelopes.
• Part of nucleoprotein core as the capsid: capsid proteins differ from matrix proteins.
• Integrate into host chromosomes in all RNA viruses: integration is a retroviral reverse-transcription event, not a matrix function.

Common Pitfalls:
Confusing matrix proteins with envelope glycoprotein spikes or with capsid proteins. Keep the three layers conceptually distinct: envelope (lipid), matrix (inner scaffold), and nucleocapsid (core).

Final Answer:
They form an inner layer that anchors and links the envelope to the nucleocapsid in enveloped viruses.