Cytokinesis in yeast can be achieved by plasma membrane ingression, which is 
dependent on actomyosin ring constriction. Inn1 presumably couples these 
processes by interaction with both the plasma membrane and the temporary 
actomyosin ring component Hof1. In addition, an actomyosin ring independent 
cytokinesis pathway exists in yeast. We here identified Cyk3, a key component of 
the alternative pathway, as a novel interaction partner of Inn1. The 
carboxy-terminal proline rich part of Inn1 binds the SH3 domains of either Cyk3 
or Hof1. Strains with truncated proteins lacking either of these SH3 domains do 
not display any severe phenotypes, but are synthetically lethal, demonstrating 
their crucial role in cytokinesis. Overexpression of CYK3 leads to an actomyosin 
ring independent recruitment of Inn1 to the bud neck, further supporting the 
significance of this interaction in vivo. Moreover, overexpression of CYK3 in a 
myo1 or an iqg1 deletion not only restores viability, but also the recruitment 
of Inn1 to the bud neck. We propose that Cyk3 is part of an actomyosin ring 
independent cytokinesis pathway, which acts as a rescue mechanism to recruit 
Inn1 to the bud neck.