Other proteins found associated with elastic fibres include vitronectin (Dahlback et al. 1989,1990, Hintner et al. 1991) and and a structurally unrelated group of proteins collectively termed microfibrillar-associated proteins (MFAPs) (Gibson et al. 1996, 2000, Abrams et al. 1995, Toyashima et al. 1999). The significance of these interactions is not well understood. Vitronectin is present in plasma, extracellular matrix, and the alpha granules of blood platelets. It has been implicated as a regulator of many processes including coagulation, fibrinolysis, pericellular proteolysis, complement dependent immune response, cell attachment and spreading (Zhuang et al. 1996). It interacts with integrins alphaVbeta1 (Marshall et al. 1995), alphaVbeta3 (Pytela et al. 1985), alphaVbeta5 (Panetti & McKeown Longo 1993) and alphaIIbBeta3 (Pytela et al. 1986) through Arg Gly Asp (RGD) cell binding sequences. The MFAPs are not a structurally related family but grouped due to their localization with microfibrils. MFAP1 was originally called 'associated microfibril protein' (AMP). It is a 54 kDa protein, processed to 32 kDa, localizing to fibrillin-containing microfibrils in several tissues including zonule fibers (Horrigan et al. 1992). MFAP3 is a 41 kDa serine-rich protein localized to zonular microfibrils, found in extracts of developing nuchal ligament, also expressed in fetal aorta and lung (Abrams et al. 1995). MFAP4 is a 29 kDa protein localized to fibrillin-containing microfibrils surrounding elastic fibers in aorta, skin and spleen (Toyoshima et al. 1999).