The relationship of the proletarian to capital is that of the sale of labour power.
Professionals are a cultural category of people who exert intellectually, in the context of certification mechanisms.
As Gramsci amply points out some workers work by hand and others by mind.
Professionals may therefore also be wage labourers if they are paid to sell their working by mind.
To the extent that the additional training received by professionals both embodies past labour and allows the exertion of labour power that is compounded and requires exceptional exertion the labour power of working class professionals is complex: an hour of it is proportionally greater in the production of value than that of the standard average hour of socially necessary labour time. It would take more engineers longer to perform an hour of nursing than a nurse. It would take more nurses longer to perform an hour of engineering than an engineer. This in part is reflected in a higher price for labour in professions, through intermediating processes such as the political contest of labourers with capital regarding the specific price of their labour.
However, some professionals exert themselves as petite-bourgeois, selling services that they nominally control.
Some professionals take on management positions and end up with class relationships on the scale of class traitor (foreman style managers) through to effectively stipendiary capitalists (CEO and paid Director style managers).
Yet other professionals control capital, and even should they engineer or nurse on the side they are primarily engaged in controlling others labour through higher management, directorial duties, or investment.
In some societies professionals have sought to control capital in general without receiving personal profit, and only receiving minimal social stipends above the average worker (ie 7-10 multiples of the average wage.) These attempts have been political, and, due to the claim that they portended an end to profit maximisation in capital movement, Veblen’s Engineers, Djilas’ New Class of nomenklatura, and Ehrenreich’s Professional Managerial Class have been considered as the possible nucleus for a post-capitalist non-socialist ruling class. All three of these theories point to use-value maximisation as the interests of this purported “class,” and Djilas almost goes so far to purport a bearer of social worth other than “exchange value,” in the form of relationships of cultural control over absolute labour time (ie the meetings will continue until morale improves). This account is weakly received in Marxism.