title: A 59 amino acid insertion increases Ca(2+) sensitivity of rbslo1, a Ca2+ -activated K(+) channel in renal epithelia.
abstract: We previously cloned a MaxiK channel alpha-subunit isoform, rbslo1, from rabbit kidney with an amino acid sequence highly homologous to mslo but with a 59 amino acid insertion between S8 and S9 (Morita et al., 1997. Am. J. Physiol. 273:F615-F624). rbslo1 activation properties differed substantially from mslo with much greater Ca2+ sensitivity, half-activation potential of -49 mV in 1 micron m Ca2+. We now report single-channel analysis of rbslo1 and delA, a construct produced by removal of the 59 amino acid insertion at site A. delA is identical to mslo from upstream of S1 to downstream of S10 with the exception of 8 amino acids. Slope of the steady-state Boltzmann voltage activation curve was 8.1 mV per e-fold change in probability of opening for both rbslo1 and delA. The apparent [Ca2+](i) properties in delA were more like mslo but the voltage-activation properties remained distinctly rbslo1. Ca2+ affinity decreased and transmembrane voltage effects on apparent Ca2+ affinity increased in delA. The differences between rbslo1 and other cloned channels appear to be localized at insertion site A with both the insertion sequence and amino acid substitutions near site A being important. The steeper activation slope makes the channel more responsive to small changes in transmembrane voltage while the insertion sequence makes the channel functional at physiological low levels of [Ca2+](i).
authors.affiliation: The Johns Hopkins University School of Medicine, Department of Physiology, Baltimore, MD 21205, USA.
authors.forename: K
authors.initials: K
authors.lastname: Hanaoka
authors.forename: J M
authors.initials: JM
authors.lastname: Wright
authors.forename: I B
authors.initials: IB
authors.lastname: Cheglakov
authors.forename: T
authors.initials: T
authors.lastname: Morita
authors.forename: W B
authors.initials: WB
authors.lastname: Guggino
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism*; D006224:Cricetinae; D004848:Epithelium / Q000378:metabolism; D015640:Ion Channel Gating; D007668:Kidney / Q000378:metabolism*; D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D008969:Molecular Sequence Data; D015221:Potassium Channels / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D024681:Potassium Channels, Calcium-Activated; D011817:Rabbits; D013329:Structure-Activity Relationship
chemical_list: D051037:Large-Conductance Calcium-Activated Potassium Channel alpha Subunits; D051036:Large-Conductance Calcium-Activated Potassium Channels; D015221:Potassium Channels; D024681:Potassium Channels, Calcium-Activated; D002118:Calcium