Protein sorting in the secretory pathway is essential for cellular compartmentalization and homeostasis in eukaryotic cells. The endoplasmic reticulum (ER) is the biosynthetic and folding factory of secretory cargo proteins. The cargo transport from the ER to the Golgi is highly selective, but the molecular mechanism for the sorting specificity is unclear. Here, we report that three ER membrane localized proteins, SUN3, SUN4 and SUN5, regulate ER sorting of leucine-rich repeat receptor kinases (LRR-RKs) to the plasma membrane. The triple mutant sun3/4/5 displays mis-sorting of these cargo proteins to acidic compartments and therefore impairs the growth of pollen tubes and the whole plant. Furthermore, the extracellular LRR domain of LRR-RKs is responsible for the correct sorting. Together, this study reports a mechanism that is important for the sorting of cell surface receptors.