Biological activities

Certain lectins are potent mitogens; PHA and con-canavalin A, for example, stimulate T cells, while pokeweed mitogen (PWM) stimulates both T and B cells. The mitogenic lectins are polyclonal activators, in that they activate lymphocytes, including memory-type cells, irrespective of their antigenic specificity. Prior to the advent of monoclonal antibodies to cell surface antigens, lectins were the major tool for

Gkil93

Glu1B5

Figure 2 Binding of carbohydrate and Ca2+ to the CRD of rat mannose-binding protein. (A) Ribbon representation of the CRD. The p strands are shown as arrows, the a helices as ribbons. (B) Close-up of sugar interacting with CRD. The equatorial 3-and 4-OH groups of the sugar form hydrogen bonds (dotted lines) with two glutamic acid and two asparagine residues, and coordination bonds (dashed lines) with Ca2*. (Reproduced with permission from Weis Wl, Drickamer K and Hendrickson WA (1992) Nature 360: 127-134 Copyright 1996 Macmillan Magazines Limited.)

Gkil93

Glu1B5

Figure 2 Binding of carbohydrate and Ca2+ to the CRD of rat mannose-binding protein. (A) Ribbon representation of the CRD. The p strands are shown as arrows, the a helices as ribbons. (B) Close-up of sugar interacting with CRD. The equatorial 3-and 4-OH groups of the sugar form hydrogen bonds (dotted lines) with two glutamic acid and two asparagine residues, and coordination bonds (dashed lines) with Ca2*. (Reproduced with permission from Weis Wl, Drickamer K and Hendrickson WA (1992) Nature 360: 127-134 Copyright 1996 Macmillan Magazines Limited.)

studies of the mechanism of mitogenic stimulation of lymphocytes.

In the presence of a mitogenic lectin, a wide variety of antigenically unrelated target cells are lvsed by cytotoxic T lymphocytes, a phenomenon known as lectin-dependent cytotoxicity (in analogy to antigen-dependent cytotoxicity). Another form of lectin-dependent cytotoxicity is the killing of tumor cells by macrophages. This activity resembles that of tumor recognizing antibodies that can induce macrophages to lyse tumors.

Several lectins, e.g. concanavalin A and wheat germ agglutinin, are toxic to mammalian cells in culture and to animals, but their toxicity is at least 1000 times lower than that of ricin and abrin. The cytotoxic properties of such lectins serve for the selection of clones of lectin-resistant cell mutants that are widely employed in studies of the genetics, biosynthesis and function of cell surface carbohydrates.

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