We have previously reported that the dynein light chain (DLC) km23-1 is required 
for Smad2-dependent TGFbeta signaling. Here we describe another member of the 
km23/DYNLRB/LC7/robl family of DLCs, termed km23-2, which is also involved in 
TGFbeta signaling. We show not only that TGFbeta stimulates the interaction of 
km23-2 (DYNLRB2) with TGFbeta receptor II (TbetaRII) but also that TGFbeta 
regulates the interaction between km23-2 and endogenous TbetaRII in vivo. In 
addition, TGFbeta treatment causes km23-2 phosphorylation, whereas a 
kinase-deficient form of TbetaRII prevents km23-2 phosphorylation. In contrast 
to the km23-1 isoform, blockade of km23-2 expression using small interfering 
RNAs (siRNAs) decreased key TGFbeta/Smad3-specific responses, including the 
induction of both plasminogen activator inhibitor-1 (PAI-1) gene expression and 
p21 protein expression. Blockade of km23-1 expression had no effect on these two 
major TGFbeta/Smad3 responses under similar conditions. Further, km23-2 was 
required for TGFbeta stimulation of Smad3-dependent Smad-binding element 
(SBE)2-Luc transcriptional activity, but not for TGFbeta stimulation of 
Smad2-dependent activin responsive element (ARE)-Lux transcriptional activity. 
In order to assess the mechanisms underlying the preferential stimulation of 
Smad3- versus Smad2-specific TGFbeta responses, immunoprecipitation (IP)/blot 
analyses were performed, which demonstrate that TGFbeta stimulated preferential 
complex formation of km23-2 with Smad3, relative to Smad2. Collectively, our 
findings indicate that km23-2 is required for Smad3-dependent TGFbeta signaling. 
More importantly, we demonstrate that km23-2 has functions in TGFbeta signaling 
that are distinct from those for km23-1. This is the first report to describe a 
differential requirement for unique isoforms of a specific DLC family in 
Smad-specific TGFbeta signaling.