Yeast Ty retrotransposons assemble into virus-like particles whose T-numbers depend on the C-terminal length of the capsid protein

The virus-like particles (VLPs) produced by the yeast Ty retrotransposons a re structurally and functionally related to retroviral cores. Using cryoele ctron microscopy (cryo-EM) and three-dimensional (3D) reconstruction, we ha ve examined the structures of VLPs assembled from full-length and truncated forms of the capsid structural protein. The VLPs are highly polydisperse i n their radius distribution. We have found that the length of the C-termina l region of the capsid structural protein dictates the T-number, and thus t he size, of the assembled particles. Each construct studied appears to asse mble into at least two or three size classes, with shorter C termini giving rise to smaller particles. This assembly property provides a model for und erstanding the variable assembly of retroviral core proteins. The particles are assembled from trimer-clustered units and there are holes in the capsi d shells. (C) 1999 Academic Press.