Is the long sentence okay?

I think you are right.

And note, signalling is spelled this way.

You could try,... in turn increased THE affinity of the extracellular domain for ligand...

If this is the conclusion supported by experimental evidence

A critical αIIbβ3 characteristic of function is that it displays bidirectional signaling, which is, not only intracellular signals imping[ing] on integrin αIIbβ3 cytoplasmic domains and [triggering] an unclasping of the intracellular and transmembrane αIIbβ3 complex, which, in turn increases the affinity of the extracellular domain for ligand (inside-out signaling), but also ligand-dependent signaling transmited into the cytoplasm to initiate the biochemical and cytoskeletal changes (outside-in signaling).

I have hi-lighted OF FUNCTION at the start of the parra. It seems superfluous to requirements. I believe the sentance should read either A critical αIIbβ3 characteristic OR A critical function of αIIbβ3 is...
however there may be some scientific meaning i am unaware of.

EDIT: perhaps its a typographical error and should read A critical αIIbβ3 characteristic or function

signaling US English
signalling British English

Oh really? But why would he want to learn improper English?

Well, to be frank with you, McTag, I can't see any reason, but you know what some people are like.

Thank you Dadpad.
Also thanks to McTag and Contrex.

I'm not clear about why using the past participle "transmitted" , but not present participle?