The membrane protein AtTLP18.3 of Arabidopsis (Arabidopsis thaliana) contains a domain of unknown function, DUF477; it forms a polysome with photosynthetic apparatuses in the thylakoid lumen. AtTLP18.3 has the molecular function of a novel acid phosphatase in the thylakoid lumen. DUF477 is accordingly renamed the thylakoid acid phosphatase domain.