Accessory proteins are involved in the signalling pathways of two-component systems. These can signal between the extracellular environment and the histidine kinase.
Accessory proteins can also form a scaffold between the histidine kinase and the response regulator. This can be used to mediate the transfer of phosphate groups between the two proteins.
Connector (accessory) proteins can transfer signalling to other two-component systems, allowing multiple to work together in a single pathway.
Allostery, a rare form of accessory proteins, is where the response regulator of one two-component system feeds in to a different two-component system.