One difference between the actions of steroid and non-steroid hormones is that steroids can act in two ways: on membrane receptors and directly on cytoplasmic or nuclear receptors; while non-steroid hormones work almost exclusively on membrane receptors.
There are three general types of membrane-bound hormone receptors:
Single Transmembrane Segment (1-TMS) Receptors
These receptors are linked to a three protein (α, β, and γ)complex that bindsGTP. The complex is linked to the membrane through hydrophobic sidechains. The α subunit actually binds the GTP and then can detach and interact with other membrane proteins (usually Adenyl or Guanlyl cyclase) and activate or inhibit them, by acting alone or in conjunction with another G Protein coupled receptor.
Most 1-TMS receptors are either Tyrosine kinase or Guanlyl cyclase coupled.
There are three major types of tyrosine kinase linked receptors. Two receptors act in concert, by binding their hormone, which induces their inside segments to dimerize and become active. The different subclasses of tyrosine kinase receptor do this in different ways. Subclass I acts simply as mentioned. Subclass II already consists of two parts, and dimerizes within itself, while subclass III uses two receptors to bind a hormone dimer, which leads to dimerization of the receptors.
