SMOOTHELIN EXPRESSION CHARACTERISTICS - DEVELOPMENT OF A SMOOTH-MUSCLE CELL IN-VITRO SYSTEM AND IDENTIFICATION OF A VASCULAR VARIANT

Recently we described a protein, smoothelin, that has been exclusively found in smooth muscle cells (SMC). The human cDNA has been cloned fr om a colon cDNA library and the putative protein sequence was deduced. Smoothelin does not belong to a known protein family but shows a part ial homology with members of the spectrin family. Transfection studies revealed that smoothelin has an affinity for actin and is either capa ble of forming filamentous structures or colocalizes with such structu res. The protein is expressed in visceral as well as vascular tissues of all vertebrate classes. A study on the distribution of smoothelin i n the vascular and placental system showed that smoothelin expression was largely restricted to the muscular pulsating blood vessels. Theref ore, we hypothesized that smoothelin is expressed in contractile SMC o nly (36, 37). No expression of smoothelin was observed in established cell lines of SMC. In tissue explants smoothelin mRNA concentration de creases to undetectable levels within 12 hours after dissection as was in general the case in primary cell cultures. Here we report on conti nued smoothelin expression for several passages observed in a human pr ostate primary cell culture system. Smoothelin was demonstrated to col ocalize with actin stress fibers but not with desmin filaments. This c ulture system offers opportunities to study the cytological localizati on of smoothelin, interactions with other proteins and should provide a system to test the promoter of the smoothelin gene. On immunoblots t he molecular weight of smoothelin differed between visceral and vascul ar smooth muscle tissue with apparent molecular weights of respectivel y 59 kDa and 94 kDa. There is no evidence for the existence of another gene coding for the 94 kDa smoothelin. Thus, posttranslational modifi cation, alternative splicing and dual promoter control are the alterna tives for the expression of two isoforms of smoothelin.