Fig. 4
From: Q&A: How do plants respond to ethylene and what is its importance?
The five ethylene receptor isoforms in Arabidopsis. The ethylene-binding domain consists of three conserved transmembrane domains at the N-terminus (represented by the vertical blue bars). The receptors fall into two subfamilies. Subfamily II receptors have a fourth transmembrane domain (dark blue) at the N-terminus, which possibly serves as a signal sequence, and a degenerate histidine kinase domain (light blue) that displays serine/threonine kinase activity in vitro. Other plant species have similar ethylene receptor isoforms