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The capsid size-determining protein Sid forms an external scaffold on phage P4 procapsids
O J Marvik1, T Dokland, R H Nøkling
1Institute of Biology, University of Oslo, Norway.
Journal of Molecular Biology
|August 4, 1995
Summary
Bacteriophage P4 uses the Sid protein to form a scaffold, creating smaller, P4-sized capsids. This scaffold guides the assembly of the N gene product (gpN) into a distinct shell structure, regulating capsid size.
Area of Science:
- Structural biology
- Virology
- Biochemistry
Background:
- Bacteriophages P2 and P4 assemble capsids from the same N gene precursor (gpN).
- P2 capsids are T=7 (60 nm, 420 subunits), while P4 capsids are T=4 (45 nm, 240 subunits).
- The P4 sid gene is known to be crucial for assembling P4-sized shells.
Purpose of the Study:
- To determine the structure of a P4 capsid assembly intermediate.
- To elucidate the role of the P4 Sid protein in capsid size determination.
Main Methods:
- Cryo-electron microscopy
- Image reconstruction
- In vivo production of P4 capsid intermediates from cloned genes
Main Results:
- Sid protein forms a P4-specific scaffold with icosahedral symmetry on procapsid-like particles.
- Sid molecules (60-120 copies) create arches interacting with gpN hexamers and trimers at 3-fold axes, forming an outer dodecahedral cage.
- The inner gpN shell is ~40 nm wide, and its hexamers are elongated and more protruding than in mature capsids.
Conclusions:
- Sid acts as a scaffold, dictating P4 capsid size by interacting with gpN subunits.
- The Sid scaffold and gpN shell structure provide insights into capsid size regulation mechanisms.
- Observed differences in hexamer structure suggest a maturational process influencing final capsid form.