Folate-binding proteins of three major classes have been observed in various bodily fluids and in the plasma membrane and cytoplasm of normal and neoplastic cells. A major class, the high-affinity folate-binding proteins, show a preferential and tight binding of folic acid relative to reduced folates and methotrexate and consist of water-soluble and membrane-associated forms. Soluble forms of the high-affinity binders are present in serum and milk and in the growth medium of certain cultured cell lines, whereas membrane-associated forms are observed on the surface of various cells and tissues. The binders in serum have no clearly defined function, whereas the milk binders serve to accumulate and stabilize reduced-folate compounds in milk and they may also facilitate the absorption of folates by the intestinal mucosa of neonates. Membrane-bound forms of high-affinity folate-binding proteins mediate the transport of folate compounds across plasma membranes and appear to utilize endocytosis as the transport mechanism. Membrane-associated high-affinity binding proteins contain covalently bound phospholipids and hydrophobic C-terminal amino acid sequences that are absent in the soluble forms. The remaining protein portions of these binders show considerable sequence homology. The second class is composed of folate-binding proteins that reside solely in the plasma membrane and are structurally and mechanistically distinct from the high-affinity binders. These proteins function in transport, exhibit varied substrate specificities that accommodate reduced-folate compounds with equal or higher affinity than folate, and do not utilize endocytosis as the mechanism for substrate internalization. The third class of folate-binding proteins consists of enzymes that reside in the cytoplasm of cells.

The interaction of the respiratory syncytial virus (RSV) Matrix (M) protein with the plasma membrane was investigated using polyclonal and monoclonal antisera raised against recombinant M expressed in bacteria. M bound mainly to the plasma membrane, although a significant proportion bound to internal membranes. However, no localisation of M with the Golgi was observed, suggesting that transport of M to the plasma membrane was independent of the transport mechanism for the viral glycoproteins. Expression from a recombinant baculovirus demonstrated the ability of M to bind membranes in the absence of viral glycoprotein expression. When cell-surface expression of the viral glycoproteins was prevented using Brefeldin A, M was still found in association with the plasma membrane, but the characteristics of M's membrane-binding ability were different to that found in untreated infected cells. In the presence of normal glycoprotein expression, M was sorted into lipid rafts and, in addition, formed structures that could only be disrupted by treatment with high salt buffers, a feature suggesting an interaction with the cytoskeleton or the formation of strong intramolecular associations. Brefeldin A prevented M from being sorted into lipid rafts or from forming strong intramolecular associations. Brefeldin A also affected the stability of M bound to the plasma membrane, as M was more readily dissociated in the presence of the inhibitor. Coexpression of M and F resulted in the incorporation of M into lipid rafts but did not cause the formation of the strong intramolecular bonds, suggesting that additional factors are required for this phenomena.

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