  We study a physical model for the formation of bud-like invaginations on
fluid membranes under tension, and apply this model to caveolae formation. We
demonstrate that budding can be driven by membrane-bound inclusions (proteins)
provided that they exert asymmetric forces on the membrane that give rise to
bending moments. In particular, Caveolae formation may not necessarily require
forces to be applied by the cytoskeleton. Our theoretical model is able to
explain several features observed experimentally in caveolae, where proteins in
the caveolin family are known to play a crucial role in the formation of
caveolae buds. These include (i) the formation of caveolae buds with sizes in
the 100nm range (ii) that a fairly large variation of bud shape is expected
(iii) that certain N and C termini deletion mutants result in vesicles that are
an order of magnitude larger. Finally, we discuss the possible origin of the
morphological striations that are observed on the surfaces of the caveolae.
