I think you misunderstand "proper noun" - those are words like places or names of people and I don't think they should be lowercased, see. e.g. the example 
> C.M.Th. Lindo, ‘Het perspectief van minister W. Sorgdrager’
> (interview), NJB 1995, afl. 1, p. 1-6
on p. 58 of the guide