Studies on the exchangeable nucleotide binding site of tubulin

An immunofluorescence study of the effects of ultraviolet radiation on the organization of microfilaments, keratin intermediate filaments, and microtubules in human keratinocytes

Indirect immunofluorescence microscopy has been used to investigate the ultraviolet (UV) radiation induced disruption of the organization of microfilaments, keratin intermediate filaments, and microtubules in cultured human epidermal keratinocytes. Following irradiation, concurrent changes in the organization of the three major cytoskeletal components were observed in cells incubated under low Ca2+ (0.15 mM) conditions. UV irradiation induced a dose-dependent condensation of keratin filaments into the perinuclear region. This collapse of the keratin network was accompanied by the reorganization of microfilaments into rings and a restricted distribution of microtubules, responses normally elicited by exposure to high Ca2+ (1.05 mM) medium. The UV induced alteration of the keratin network appears to disrupt the interactions between keratin and actin, permitting the reorganization of actin filaments in the absence of Ca2+ stimulation. In addition to the perinuclear condensation of keratin filaments, UV irradiation inhibits the Ca2+ induced formation of keratin alignments at the membrane of apposed cells if UV treatment precedes exposure to high Ca2+ medium. Incubation of keratinocytes in high Ca2+ medium for 24 hours prior to irradiation results in the stabilization of membrane associated keratin alignments and a reduced susceptibility of cytoplasmic keratin filaments to UV induced disruption. Unlike results from investigations with isogenic skin fibroblasts, no UV induced disassembly of microtubules was discernible in irradiated human keratinocytes.

A model of the nucleotide-binding site in tubulin

Tubulin uses GTP to regulate microtubule assembly and is thought to be a member of a class of GDP/GTP-binding proteins (G-proteins) as defined by Hughes [(1983) Febs Lett. 164, 1-8]. How tubulin is structurally related to G-proteins is not known. We use a synthesis of sequence comparisons between tubulin, other G-proteins, and ADP/ATP-binding proteins and topological arguments to identify potential regions involved in nucleotide binding. We propose that the nucleotide-binding domain in the beta-subunit of tubulin is an alpha/beta structure derived from amino acid residues approximately 60-300. Five peptide sequences are identified which we suggest exist as 'loops' that extend from beta-strands and connect alpha-helices in this structure. We argue that GDP binds to four of the five loops in an Mg2+-independent manner while GTP binds in an Mg2+-dependent manner to a different combination of four loops. We propose that this switch between loops upon GTP binding induces a conformational change essential for microtubule assembly.