Main research topics and results

1. Regulation of membrane markers and growth factors during differentiation and development of enterocytes and thymocytes. (i) Much higher expression of TGF-a mRNA than EGF mRNA observed in developing small intestine suggests that TGF-a is an integral physiological regulator of growth of the small intestine during ontogeny. (ii) Increase in the relative number of mature thymocytes by the antiglucocorticoids mifepristone and onapristone indicate a negative regulation of the development of T-lymphocyte precursors at the level of dividing precursors. (iii) Lower expression of fucosylated structures on brush-border membranes of enterocytes of adult germ-free rats as compared with conventional rats suggests the importance of terminally bound fucose for bacterial colonization. (iv) Surface of mannose-specific lectin of Galanthus nivalis discriminates between immature and mature thymocytes. The expression of the target surface structure on thymocytes is multi-modal.

2. Structure and molecular mechanisms of function of iontransport systems. (i) The NHAl has been cloned and sequenced in Saccharomyces cerevisiae. Its molecular and functional characterization revealed that it encodes a sodium/proton antiport with dual function. It may be involved not only in the regulation of the internal concentrations of K+, toxic Na+, Li+, and other alkaline cations, but also in buffering of cytosolic pH. (ii) Plasma membrane H+ATPase of S. cerevisiae plays the most important role in acidification mechanisms in yeast following addition of glucose or other metabolizable sugars. The H+ATPase is activated through phosphorylation of some amino acid residues near the Cterminus. The process starts with a specific sensor (most probably activated by fructose 6phosphate) and involves one of two G proteins (also defined here), which activate phospholipase C and subsequently protein kinase C, instrumental in ATPase phosphorylation. The second source of acidity is the glycolytic production of (phosphorylated) organic acids ( a process most pronounced with glucose and fructose but very little with galactose or maltose. (iii) ATP hydrolysis by Na+/K+ATPase proceeds via interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) affinity substrate binding sites. The interacting high and lowaffinity ATPbinding sites resides on different catalytic subunits, and active Na+/K+ transport requires cooperation of the catalytic subunits. In addition, all available data are consistent with the hypothesis that Na+/K+ATPase in the plasma membrane is an ((()2 diprotomer and works as a functional dimer. A key role in the Na+/K+ATPase function is played by the large cytoplasmic loop between the fourth and fifth transmembrane segments, where the high affinity ATPbinding site is localized.

3. Biogenesis and regulation by proteolytic degradation of amino acid and sugar transporters.
(i) The functional heterologous expression of Candida albicans Can l transporter in Saccharomyces cerevisiae was shown to depend upon an operational secretory pathway in S. cerevisiae Moreover, Shr3p, an ER-resident protein that participates in specific packaging of S. cerevisiae amino acid transporters into ER-derived transport vesicles, is also required. (ii) Plasma membrane galactose-specific transporter Ga12 is internalized via the endocytotic pathway and subsequently de-graded in the vacuole in response to glucose addition to galactosegrown cells. The ubiquitination of Ga12, involving transfer of ubiquitin moieties to lysine residues of the Gal2 through the El-E2-E3 enzyme thioester cascade, signals its degradation pathway. Of 19 known El-E2-E3 protein components of the ubiquitination machinery, only four (Ubcl,-4,-5 and Rsp5/Npil) are required for Gal2 degradation. (iii) Can l transporter of C. albicans expressed in S. cerevisiae also appears to be degraded in the vacuole in a ubiquitindependent manner.

Publications