Microbial lectin cofunction with lytic activities as a model for a general basic lectin role

Lectins are ubiquitous proteins, which exhibit a specific and reversible sugar-binding activity. They react with glycosylated macromolecules and cells and may coaggregate them and lead to their lysis or alterations. Various lectin biological effects are well known, but their basic biological function is considered as yet unknown. In the present review, an experimental evidence and theoretical considerations are forwarded for supporting our suggestion that the general basic lectin or lectinoid (lectin-like protein) function in microorganisms, plants and animals is a cofunction enabling the activities of key lytic enzymes (lysins: glycosidases, proteases, esterases, phosphatases, hemolysin, etc.). The lectin service is: homing onto glycosylated receptors, anchoring to them and induction of cooperative conformational effects which enable their counterpart lysin activity on exogenous or endogenous target molecules and cells. The 'lectin-lysin' pair may reside in the same molecule, or in linked subunits. It may also be formed by cofunction of two separate entities originating from one or two (homogenous or heterogenous) cell sources. The lectin and lysin may be free or cell-bound components located intra or extracellularly. The final result of their cofunction is practically irreversible; either cell and macromolecule lysis for nutrition, homeostasis and protection or cell alteration, reorganization and new productivity. Our suggestion emphasizes the prominent analogy of lectins to lytic enzyme positioning sites (LEPS), immunoglobulins and polypeptide hormones. The lectin analogy to LEPS and immunoglobulins is exhibited in the lectin-dependent cell and macromolecule lysis for nutritional and homeostatic purposes or for protection, respectively. The hormone-like lectin activity is exhibited in the lectin-dependent cell alterations. In addition to similar functions and effects, the analogy also includes the properties and behavior of these proteins. The suggested hypothesis is based on experimental evidence from microorganisms, plants and animals. It envisions the lectin and lectinoid function in cell attacks on glycosylated molecules or cells, cell-substratum and cell-cell interactions (fusion, invasion, etc.), cell transformation and formation of special structures. All of them according to a developmental program, or special (especially unfavourable) environmental conditions. The lectin resistance to proteolysis and unfavourable pH or temperature is in accord with the suggested hypothesis.