The matrix protein (M) of respiratory syncytial virus (RSV), the prototype viral member of the Pneumovirinae (family Paramyxoviridae, order Mononegavirales), has been crystallized and the structure determined to a resolution of 1.6 Å. The structure comprises 2 compact -rich domains connected by a relatively unstructured linker region. Due to the high degree of side-chain order in the structure, an extensive contiguous area of positive surface charge covering Ϸ600 Å 2 can be resolved. This unusually large patch of positive surface potential spans both domains and the linker, and provides a mechanism for driving the interaction of the protein with a negatively-charged membrane surface or other virion components such as the nucleocapsid. This patch is complemented by regions of high hydrophobicity and a striking planar arrangement of tyrosine residues encircling the C-terminal domain. Comparison of the RSV M sequence with other members of the Pneumovirinae shows that regions of divergence correspond to surface exposed loops in the M structure, with the majority of viral species-specific differences occurring in the N-terminal domain.CD ͉ crystal structure ͉ respiratory syncytial virus ͉ sequence alignment R espiratory syncytial virus (RSV) is the prototype member of the Pneumovirinae, a subfamily of the Paramyxoviridae (order Mononegavirales). Morphologically, the extracellular virion consists of a lipid bilayer envelope, within which are embedded 3 glycoproteins, 2 of which (F and G) are important in cell attachment and viral entry into target cells. The third, the SH protein, contributes to pathology in the host (1). Internally, virions contain helical nucleocapsids that consist of N protein tightly bound to the negative-sense nonsegmented genomic RNA. The nucleocapsid in turn is associated with components of the viral RNA-dependent RNA polymerase (L, P, M2-1, and M2-2 proteins), forming the holo-nucleocapsid (2-4). Between the holo-nucleocapsid and the outer envelope there is a layer of matrix protein (M), which is associated peripherally with the membrane (5). The other family members of the Mononegavirales (Rhabdoviridae, Filoviridae, and Bornaviridae) all subscribe to this basic arrangement of the virion, although the overall morphology can vary between the families. For example, Paramyxoviridae virions are pleiomorphic, whereas the Rhabdoviridae have a regular bullet shape structure, and the Filoviridae have a more filamentous shape. Extracellular RSV virions form by a budding process that occurs at the plasma membrane within specialized lipid domains (5, 6) and M appears to drive the final assembly process, which is the incorporation of the holo-nucleocapsid and initiation of the budding process (7,8). Before budding, there is a coordinated assembly of viral components; and it is evident that the glycoproteins and M proteins are important determinants of the location on the plasma membrane at which the virus buds (9). It is also possible that the interaction between M and the glycoproteins, possibly mediat...