<p>The majority of proteins within mitochondria and chloroplasts are nuclear-encoded – they are expressed by the host and are imported into the organelle. Proteins meant for the organelle are usually marked by a targeting sequence at one end, also known as a transit peptide, which directs the protein to its destination after which it is cleaved. </p>
<p>This is no different with UCYN-A: Coele et al [1] in their 2024 study used proteomics to find proteins encoded by the host and imported into the nitroplast. Upon examining these protein sequences, they noticed that many of them possess characteristics of organellar import – most of them possess a C-terminal 120 amino acid extension compared to their orthologues. This extension is reminiscent of targeting sequences known to exist in mitochondrial [2] and chloroplastic [3] imported proteins. They termed the putative targeting sequence uTP (UCYN-A Transit Peptide, with lowercase “u” to differentiate it from uridine triphosphate).
