β1integrins are distributed in adhesion structures with fibronectin and caveolin and in coated pits

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Abstract
Integrins are found in adhesion structures, which link the extracelullar matrix to cytoskeletal proteins. Here, we attempt to further define the distribution of β1integrins in the context of their association with matrix proteins and other cell surface molecules relevant to the endocytic process. We find that β1integrins colocalize with fibronectin in fibrillar adhesion structures. A fraction of caveolin is also organized along these adhesion structures. The extracellular matrix protein laminin is not concentrated in these structures. The α4β1integrin exhibits a distinct distribution from other β1integrins after cells have adhered for 1 h to extracellular matrix proteins but is localized in adhesion structures after 24 h of adhesion. There are differences between the fibronectin receptors: α5β1integrins colocalize with adaptor protein-2 in coated pits, while α4β1integrins do not. This parallels our earlier observation that of the two laminin receptors, α1β1and α6β1, only αaβ1integrins colocalize with adaptor protein-2 in coated pits. Calcium chelation or inhibition of mitogen-activated protein kinase kinase, protein kinase C, or src did not affect localization of α1β1and α5β1integrins in coated pits. Likewise, the integrity of coated-pit structures or adhesion structures is not required for integrin and adaptor protein-2 colocalization. This suggests a robust and possibly constitutive interaction between these integrins and coated pits.Key words: adhesion, endocytosis, extracellular matrix, microscopy, confocal, signalling.