Crosslinking of alpha-crystallin with bisimidoesters. Evidence for polyamidine formation at pH8 from an increase in positive charges on the polypeptide chains

Modification of sodium and gating currents by amino group specific cross-linking and monofunctional reagents

To test the possible role of lysine residues in Na channel function the effects of several imidoesters on Na and gating currents were studied in voltage-clamped single frog nerve fibers. Mono- and bisimidoesters were used. These reagents modify amino groups exclusively and do not change the net charge. The three bisimidoesters used easily introduce cross-links between neighboring amino groups. Their structure is almost identical; only the length of the spacers between the two amino-reactive groups is different. An irreversible reduction of Na currents and gating currents was observed with the longest (dimethyl suberimidate [DMS]) and the shortest (dimethyl adipimidate [DMA]) of the cross-linkers used. Of the three cross-linking reagents only the shortest made Na current inactivation slow and incomplete. The steady-state inactivation curve, h infinity (E), was shifted by greater than 25 mV in the hyperpolarizing direction by each of the reagents. The voltage dependence of activation, however, remained unchanged. Furthermore, the effects of two different monoimidoesters (ethyl acetimidate [EAI] and isethionyl acetimidate [IAI]) on gating currents were tested. EAI can penetrate a membrane, whereas IAI is membrane impermeant. IAI was almost without effect, whereas EAI caused a considerable reduction of the gating currents. EAI and DMS reduced the Qoff/Qon ratio without affecting the decay of the Na currents. The results show that lysine residues are critically involved in Na channel gating.

Permanent suppression of phase separation cataract in calf lens using amine modification agents

Low temperature induced opacification (cold cataract) of the nucleus of young mammalian lenses is associated with a phase separation of proteins in the lens cell cytoplasm. Calf lenses were treated with a variety of imido-esters and N-hydroxysuccinimide-esters, which react specifically with amino groups. Many potent inhibitors of phase separation cataract were identified which lower the opacification temperature by 6 degrees C or more. Lenses generally remain clear, colorless and soft. Furthermore, suppression of the cold cataract temperature is permanent upon removal of excess reagent.

The reaction of citraconic anhydride with bovine alpha-crystallin lysine residues. Surface probing and dissociation-reassociation studies

Citraconic anhydride reacts readily with alpha-crystallin's lysine residues at pH 7.4. Upon addition of 2 equivalents of citraconic anhydride per equivalent lysine, 24% of the lysine residues were modified without disrupting the native quaternary structure. Further citraconylation led to dissociation into 10 S aggregates. Complete dissociation into subunits (1.4 S) occurred after adding 100 equivalents of citraconic anhydride, resulting in 98% modification. Decitraconylation did not lead to reaggregates identical with the native ones. The unmodified and the once and twice citraconylated alpha-crystallin subunits were discerned by isoelectric focusing according to their theoretical isoelectric points. In the native alpha-crystallin aggregates, nearly all B chains and approx. 60% of the A chains were found to possess at least one surface-exposed lysine residue. No differences between the susceptibilities to citraconylation of the in vivo deamidated (A1 and B1) and the de novo synthesized (A2 and B2) subunits were found. These results support the three-layer spherical assembly model for the alpha-crystallin quaternary structure.

The quaternary structure of bovine alpha-crystallin. Chemical crosslinking with bifunctional imido esters

Reaction of calf alpha-crystallin, which consists of about 30 A and 10 B subunits, with various bisimido esters at pH 8.0-8.5 and room temperature leads to inter-subunit crosslinking. Little difference is observed in the efficiency of crosslinking by reagents of a homologous series with different lengths (from C6 to C12). At any stage of the reaction an exponential decrease is found in the amounts of crosslinked dimer, trimer, tetramer etc., with no preference for the formation of any particular oligomer, suggesting that all subunits on the surface of the spherical alpha-crystallin molecule are in quasi-equivalent positions. Molecular weight analysis by docdecylsulfate gel electrophoresis and reversible crosslinking show that both A and B subunits occur in an apparently constant ratio in the crosslinked oligomers, from which we infer that both types of subunits are randomly arranged on the surface of the alpha-crystallin molecule. Not all of the subunits can form inter-chain crosslinks, as a limiting value of about 60% of the subunits is found in oligomeric form. A simple explanation could be a core of subunits which is inaccessible to certain chemical reagents, although other alternatives are also discussed.