The perinuclear actin cap is an important cytoskeletal structure that regulates 
nuclear morphology and re-orientation during front-rear polarisation. The 
mechanisms regulating the actin cap are currently poorly understood. Here, we 
demonstrate that STEF/TIAM2, a Rac1 selective guanine nucleotide exchange 
factor, localises at the nuclear envelope, co-localising with the key 
perinuclear proteins Nesprin-2G and Non-muscle myosin IIB (NMMIIB), where it 
regulates perinuclear Rac1 activity. We show that STEF depletion reduces apical 
perinuclear actin cables (a phenotype rescued by targeting active Rac1 to the 
nuclear envelope), increases nuclear height and impairs nuclear re-orientation. 
STEF down-regulation also reduces perinuclear pMLC and decreases 
myosin-generated tension at the nuclear envelope, suggesting that STEF-mediated 
Rac1 activity regulates NMMIIB activity to promote stabilisation of the 
perinuclear actin cap. Finally, STEF depletion decreases nuclear stiffness and 
reduces expression of TAZ-regulated genes, indicating an alteration in 
mechanosensing pathways as a consequence of disruption of the actin cap.