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Paper text:
/ a22c / w184a / m185a or p17c / r18l / t19c / w184a / m185a mutations ) were prepared by sequential dialysis of purified protein .
crystals were obtained by the sitting - drop vapor diffusion method in tris - buffered precipitant solutions containing polyethylene glycol and sodium iodide .
molecular replacement phasing , model building , and crystallographic refinement were performed with the programs molrep , coot , and phenix .
the capsid model was built by manual rigid - body docking of the high - resolution structures of the 5-fold symmetric ntd ring ( pdb code 3p05 ) , the 6-fold symmetric ntd ring ( 3h47 ) , and 2-fold symmetric ctd dimers ( 2kod and 1a43 ) into a geometric fullerene cone model .
the mature capsids of human immunodeficiency virus type 1 ( hiv-1 ) and other retroviruses are fullerene shells , composed of the viral ca protein , that enclose the viral genome and facilitate its delivery into new host cells1 . 
 retroviral ca proteins contain independently - folded n - terminal and c - terminal domains ( ntd and ctd ) that are connected by a flexible linker24 . 
 the ntd forms either hexameric or pentameric rings , whereas the ctd forms symmetric homodimers that connect the rings into a hexagonal lattice3,513 . 
 we previously used a disulfide crosslinking strategy to enable isolation and crystallization of soluble hiv-1 ca hexamers11,14 . by the same approach 
 , we have now determined the x - ray structure of the hiv-1 ca pentamer at 2.5  resolution . 
 two mutant ca proteins with engineered disulfides at different positions ( p17c / t19c and n21c / a22c ) converged onto the same quaternary structure , indicating that the disulfide - crosslinked proteins recapitulate the structure of the native pentamer . 
 assembly of the quasi - equivalent hexamers and pentamers requires remarkably subtle rearrangements in subunit interactions , and appears to be controlled by an electrostatic switch that favors hexamers over pentamers . 
 this study completes the gallery of sub - structures describing the components of the hiv-1 capsid and enables atomic level modeling of the complete capsid . 
 rigid - body rotations around two assembly interfaces appear sufficient to generate the full range of continuously varying lattice curvature in the fullerene cone .