<p><em>Immature retroviral particles are assembled by self-association  of the structural  polyprotein precursor Gag. During maturation the Gag  polyprotein is proteolytically  cleaved, yielding mature structural  proteins, matrix (MA), capsid (CA), and nucleocapsid (NC), that  reassemble into a mature viral particle. Proteolytic cleavage causes the  N terminus of CA to fold back to form a &beta;-hairpin, anchored by an  internal salt bridge between the N-terminal proline and the inner  aspartate. Using an in vitro assembly system of capsid-nucleocapsid  protein (CANC), we studied the formation of virus-like particles (VLP)  of a gammaretrovirus, the xenotropic murine leukemia virus (MLV) related  virus (XMRV). We show here that, unlike other retroviruses, XMRV CA and  CANC do not assemble tubular particles characteristic of mature  assembly. The prevention of &beta;-hairpin formation by the deletion of  either the N-terminal proline or 10 initial amino acids enabled the  assembly of &#8710;ProCANC or &#8710;10CANC into immature-like spherical particles.  Detailed three-dimensional (3D) structural analysis of these particles  revealed that below a disordered N-terminal CA layer, the C terminus of  CA assembles a typical immature lattice, which is linked by rod-like  densities with the RNP.</em></p>