To understand the effects of a crystalline protein layer on bilayer properties, we studied the mechanical properties of avidin- and streptavidin-coated giant lipid bilayer vesicles. The giant vesicles (20-60 mum) are made from a mixture of SOPC and biotinylated phospholipids via electroformation. Using micropipet manipulation, we showed that the presence of a monomolecular layer of noncrystalline avidin on the vesicle surface increases the membrane bending rigidity but does not significantly alter the elastic area expansion modulus of the vesicle. When the vesicles were coated with streptavidin, the protein crystallizes on the bilayer surface, resulting in a rigid polycrystalline membrane. These vesicles display unique roughened spherical or prolate ellipsoidal shapes, depending on the differences in crystal morphologies. Upon aspiration with micropipets, the vesicles first showed rapid permanent deformation at low strain, followed by a slower viscoelastic response above a certain threshold. Despite their extremely rigid appearance, the existence of a polycrystalline shell does not increase the toughness of streptavidin-coated vesicles above that of uncoated vesicles. The origin of these properties can be traced to the unique ligand-receptor interactions between streptavidin and biotinylated phospholipids in the bilayer membrane. The findings offer greater understandings of complex phenomena involving crystalline protein layers on the surface of cell membranes, in addition to providing information for the development of various applications involving the immobilization of functionalized molecules on lipid bilayer substrates.