Taurine inhibition involves the CT of Cx26. Hemichannel activity in liposomes is assessed as permeability to urea/sucrose by TSF. (A) Effect of the AS taurine on activity of carboxyl-terminal epitope-tagged (_T) homomeric (Cx32_T and Cx26_T) and heteromeric (Cx26/Cx32_T and Cx26_T/Cx32) hemichannels. Only heteromeric hemichannels in which the Cx26CT is not tagged were AS sensitive; a tag on Cx26CT eliminated the AS sensitivity of homomeric Cx26_T and heteromeric Cx26_T /Cx32 hemichannels. Previous work showed that wild-type (untagged) Cx26-containing hemichannels were AS sensitive, and that homomeric Cx32 hemichannels were not (Bevans and Harris, 1999; Locke et al., 2004b; Tao and Harris, 2004). In this and subsequent TSF figures, the results for each experimental condition with taurine were normalized to its own control experiment without taurine (n = 6–9). (B) Effect of tag cleavage (_Tc), which leaves four additional amino acids (LVPR) at the carboxyl terminus, on sensitivity to AS. Cleavage restores inhibition to heteromeric Cx26_Tc/Cx32 hemichannels. AS sensitivity of all other hemichannel compositions was unaffected by tag cleavage. Surprisingly, the activity of homomeric Cx26_Tc hemichannels was unaffected by AS (n = 5–8). (C) AS narrows rather than closes Cx26_Tc hemichannels. In TSF, liposome permeability to urea but not sucrose forms a band of intermediate density. (Left to right) TSF gradients with liposomes formed in the absence of connexin (showing upper band only), liposomes containing Cx26_Tc hemichannels, and liposomes containing Cx26_T hemichannels, all exposed to 10 mM AS. For Cx26_Tc, the lower band density is shifted upwards compared with Cx26_T hemichannels. Therefore, AS narrows rather than closes Cx26_Tc hemichannels but has no effect on activity in TSF (B) or permeation (C) of Cx26_T hemichannels. None of the other channel compositions (tagged or tag-cleaved) was affected in this way by gradient AS. In A and B, the error bars are standard errors of the mean.