Fig 3. Shape of a bud that minimizes the Helfrich bending energy.

The blue line, which represents the bare lipid bilayer membrane, has the shape of catenoid of revolution. The heavy red line, which represents the lipid bilayer attached to a curved layer of capsid proteins, has the shape of spherical cap. The interface is a circle. The boundary between the two bilayers and the center of the sphere spans a cone with aperture angle 2α. For purposes of illustration, RNA genome molecules associated with the bud are indicated by a black line. Two capsid proteins diffusing along the lipid bilayer are indicated as two red bars associated both with the membrane and an RNA genome molecule. A curvilinear coordinate system (s, φ) is indicated where s measures the shortest arc distance between a point and the cross-section with minimum diameter (s = 0). The value of s ranges from s_M > 0 to s_m < 0. Finally, φ is the azimuthal angle of the circle on the surface perpendicular to the central axis on which the point is located.