Q:

Essential by something

A group of proteins, summarised as the sarcomeric cytoskeleton, is essential for the ordered assembly of actin and myosin filaments into sarcomeres, by combining architectural, mechanical and signalling functions.

Does it mean that SC is essential for assembly by combining or it can also mean that SC is essential by combining for the assembly?

A:

It means that SC is essential for the ordered assembly of actin and myosin filaments into sarcomeres. 
The last part means that SC uses the process of combining architectural, mechanical and signalling functions to assemble the above into sarcomeres.

