Inhibition of flippase-like activity by tubulin regulates phosphatidylserine exposure in erythrocytes from hypertensive and diabetic patients.

Plasma membrane tubulin is an endogenous regulator of P-ATPases and the unusual accumulation of tubulin in the erythrocyte membrane results in a partial inhibition of some their activities, causing hemorheological disorders like reduced cell deformability and osmotic resistance. These disorders are of particular interest in hypertension and diabetes, where the abnormal increase in membrane tubulin may be related to the disease development. Phosphatidylserine (PS) is more exposed on the membrane of diabetic erythrocytes than in healthy cells. In most cells, PS is transported from the exoplasmic to the cytoplasmic leaflet of the membrane by lipid flippases. Here, we report that PS is more exposed in erythrocytes from both hypertensive and diabetic patients than in healthy erythrocytes, which could be attributed to the inhibition of flippase activity by tubulin. This is supported by: (i) the translocation rate of a fluorescent PS analog in hypertensive and diabetic erythrocytes was slower than in healthy cells, (ii) the pharmacological variation of membrane tubulin in erythrocytes and K562 cells was linked to changes in PS translocation and (iii) the P-ATPase-dependent PS translocation in inside-out vesicles (IOVs) from human erythrocytes was inhibited by tubulin. These results suggest that tubulin regulates flippase activity and hence, the membrane phospholipid asymmetry.

Post-translational incorporation of 3,4-dihydroxyphenylalanine into the C terminus of α-tubulin in living cells.

The C-terminal tyrosine (Tyr) of the α-tubulin chain is subjected to post-translational removal and readdition in a process termed the "detyrosination/tyrosination cycle". We showed in previous studies using soluble rat brain extracts that l-3,4-dihydroxyphenylalanine (l-Dopa) is incorporated into the same site as Tyr. We now demonstrate that l-Dopa incorporation into tubulin also occurs in living cells. We detected such incorporation by determining the "tyrosination state" of tubulin before and after incubation of cells in the presence of l-Dopa. The presence of a tubulin isospecies following l-Dopa incubation that was not recognized by antibodies specific to Tyr- and deTyr-tubulin was presumed to reflect formation of Dopa-tubulin. l-Dopa was identified by HPLC as the C-terminal compound bound to α-tubulin. l-Dopa incorporation into tubulin was observed in Neuro 2A cells and several other cell lines, and was not due to de novo protein biosynthesis. Dopa-tubulin had microtubule-forming capability similar to that of Tyr- and deTyr-tubulin. l-Dopa incorporation into tubulin did not notably alter cell viability, morphology, or proliferation rate. CAD cells (a neuron-like cell line derived from mouse brain) are easily cultured under differentiating and nondifferentiating conditions, and can be treated with l-Dopa. Treatment of CAD cells with l-Dopa and consequent increase in l-Dopa-tubulin resulted in reduction of microtubule dynamics in neurite-like processes.