title: A cataract-causing connexin 50 mutant is mislocalized to the ER due to loss of the fourth transmembrane domain and cytoplasmic domain.
abstract: Mutations in the eye lens gap junction protein connexin 50 cause cataract. Earlier we identified a frameshift mutant of connexin 50 (c.670insA; p.Thr203AsnfsX47) in a family with autosomal recessive cataract. The mutant protein is smaller and contains 46 aberrant amino acids at the C-terminus after amino acid 202. Here, we have analysed this frameshift mutant and observed that it localized to the endoplasmic reticulum (ER) but not in the plasma membrane. Moreover, overexpression of the mutant resulted in disintegration of the ER-Golgi intermediate compartment (ERGIC), reduction in the level of ERGIC-53 protein and breakdown of the Golgi in many cells. Overexpression of the frameshift mutant partially inhibited the transport of wild type connexin 50 to the plasma membrane. A deletion mutant lacking the aberrant sequence showed predominant localization in the ER and inhibited anterograde protein transport suggesting, therefore, that the aberrant sequence is not responsible for improper localization of the frameshift mutant. Further deletion analysis showed that the fourth transmembrane domain and a membrane proximal region (231-294 amino acids) of the cytoplasmic domain are needed for transport from the ER and localization to the plasma membrane. Our results show that a frameshift mutant of connexin 50 mislocalizes to the ER and causes disintegration of the ERGIC and Golgi. We have also identified a sequence of connexin 50 crucial for transport from the ER and localization to the plasma membrane.
authors.affiliation: Centre for Cellular and Molecular Biology, Council of Scientific and Industrial Research, Uppal Road, Hyderabad 500 007, India.
authors.forename: Madhavi Latha
authors.initials: ML
authors.lastname: Somaraju Chalasani
authors.forename: Madhavi
authors.initials: M
authors.lastname: Muppirala
authors.forename: Surya Prakash
authors.initials: SP
authors.lastname: G Ponnam
authors.forename: Chitra
authors.initials: C
authors.lastname: Kannabiran
authors.forename: Ghanshyam
authors.initials: G
authors.lastname: Swarup
keywords: Cataract; Cataract-causing mutant; Connexin 50; Cx50, connexin 50; Defective transport; ER, endoplasmic reticulum; ERGIC, ER-Golgi intermediate compartment; Protein targeting; TGN, trans-Golgi network